For us deep studying practitioners, the world is – not flat, however – linear, principally. Or piecewise linear. Like different
linear approximations, or perhaps much more so, deep studying will be extremely profitable at making predictions. However let’s
admit it – typically we simply miss the fun of the nonlinear, of fine, outdated, deterministic-yet-unpredictable chaos. Can we
have each? It appears to be like like we will. On this put up, we’ll see an software of deep studying (DL) to nonlinear time collection
prediction – or reasonably, the important step that predates it: reconstructing the attractor underlying its dynamics. Whereas this
put up is an introduction, presenting the subject from scratch, additional posts will construct on this and extrapolate to observational
datasets.
What to anticipate from this put up
In his 2020 paper Deep reconstruction of unusual attractors from time collection (Gilpin 2020), William Gilpin makes use of an
autoencoder structure, mixed with a regularizer implementing the false nearest neighbors statistic
(Kennel, Brown, and Abarbanel 1992), to reconstruct attractors from univariate observations of multivariate, nonlinear dynamical programs. If
you’re feeling you fully perceive the sentence you simply learn, you could as properly immediately leap to the paper – come again for the
code although. If, alternatively, you’re extra accustomed to the chaos in your desk (extrapolating … apologies) than
chaos idea chaos, learn on. Right here, we’ll first go into what it’s all about, after which, present an instance software,
that includes Edward Lorenz’s well-known butterfly attractor. Whereas this preliminary put up is primarily alleged to be a enjoyable introduction
to a captivating subject, we hope to observe up with functions to real-world datasets sooner or later.
Rabbits, butterflies, and low-dimensional projections: Our drawback assertion in context
In curious misalignment with how we use “chaos” in day-to-day language, chaos, the technical idea, could be very completely different from
stochasticity, or randomness. Chaos might emerge from purely deterministic processes – very simplistic ones, even. Let’s see
how; with rabbits.
Rabbits, or: Delicate dependence on preliminary circumstances
You could be accustomed to the logistic equation, used as a toy mannequin for inhabitants progress. Typically it’s written like this –
with (x) being the scale of the inhabitants, expressed as a fraction of the maximal measurement (a fraction of doable rabbits, thus),
and (r) being the expansion price (the speed at which rabbits reproduce):
[
x_{n + 1} = r x_n (1 – x_n)
]
This equation describes an iterated map over discrete timesteps (n). Its repeated software leads to a trajectory
describing how the inhabitants of rabbits evolves. Maps can have fastened factors, states the place additional operate software goes
on producing the identical consequence eternally. Instance-wise, say the expansion price quantities to (2.1), and we begin at two (fairly
completely different!) preliminary values, (0.3) and (0.8). Each trajectories arrive at a hard and fast level – the identical fastened level – in fewer
than 10 iterations. Have been we requested to foretell the inhabitants measurement after 100 iterations, we might make a really assured
guess, regardless of the of beginning worth. (If the preliminary worth is (0), we keep at (0), however we will be fairly sure of that as
properly.)
Determine 1: Trajectory of the logistic map for r = 2.1 and two completely different preliminary values.
What if the expansion price have been considerably increased, at (3.3), say? Once more, we instantly examine trajectories ensuing from preliminary
values (0.3) and (0.9):
Determine 2: Trajectory of the logistic map for r = 3.3 and two completely different preliminary values.
This time, don’t see a single fastened level, however a two-cycle: Because the trajectories stabilize, inhabitants measurement inevitably is at
one in every of two doable values – both too many rabbits or too few, you possibly can say. The 2 trajectories are phase-shifted, however
once more, the attracting values – the attractor – is shared by each preliminary circumstances. So nonetheless, predictability is fairly
excessive. However we haven’t seen all the things but.
Let’s once more improve the expansion price some. Now this (actually) is chaos:
Determine 3: Trajectory of the logistic map for r = 3.6 and two completely different preliminary values, 0.3 and 0.9.
Even after 100 iterations, there isn’t a set of values the trajectories recur to. We are able to’t be assured about any
prediction we’d make.
Or can we? In spite of everything, we have now the governing equation, which is deterministic. So we should always have the ability to calculate the scale of
the inhabitants at, say, time (150)? In precept, sure; however this presupposes we have now an correct measurement for the beginning
state.
How correct? Let’s examine trajectories for preliminary values (0.3) and (0.301):
Determine 4: Trajectory of the logistic map for r = 3.6 and two completely different preliminary values, 0.3 and 0.301.
At first, trajectories appear to leap round in unison; however throughout the second dozen iterations already, they dissociate extra and
extra, and more and more, all bets are off. What if preliminary values are actually shut, as in, (0.3) vs. (0.30000001)?
It simply takes a bit longer for the disassociation to floor.
Determine 5: Trajectory of the logistic map for r = 3.6 and two completely different preliminary values, 0.3 and 0.30000001.
What we’re seeing right here is delicate dependence on preliminary circumstances, an important precondition for a system to be chaotic.
In an nutshell: Chaos arises when a deterministic system exhibits delicate dependence on preliminary circumstances. Or as Edward
Lorenz is claimed to have put it,
When the current determines the long run, however the approximate current doesn’t roughly decide the long run.
Now if these unstructured, random-looking level clouds represent chaos, what with the all-but-amorphous butterfly (to be
displayed very quickly)?
Butterflies, or: Attractors and unusual attractors
Really, within the context of chaos idea, the time period butterfly could also be encountered in several contexts.
Firstly, as so-called “butterfly impact,” it’s an instantiation of the templatic phrase “the flap of a butterfly’s wing in
_________ profoundly impacts the course of the climate in _________.” On this utilization, it’s principally a
metaphor for delicate dependence on preliminary circumstances.
Secondly, the existence of this metaphor led to a Rorschach-test-like identification with two-dimensional visualizations of
attractors of the Lorenz system. The Lorenz system is a set of three first-order differential equations designed to explain
atmospheric convection:
[
begin{aligned}
& frac{dx}{dt} = sigma (y – x)
& frac{dy}{dt} = rho x – x z – y
& frac{dz}{dt} = x y – beta z
end{aligned}
]
This set of equations is nonlinear, as required for chaotic habits to seem. It additionally has the required dimensionality, which
for easy, steady programs, is a minimum of 3. Whether or not we really see chaotic attractors – amongst which, the butterfly –
is dependent upon the settings of the parameters (sigma), (rho) and (beta). For the values conventionally chosen, (sigma=10),
(rho=28), and (beta=8/3) , we see it when projecting the trajectory on the (x) and (z) axes:
Determine 6: Two-dimensional projections of the Lorenz attractor for sigma = 10, rho = 28, beta = 8 / 3. On the correct: the butterfly.
The butterfly is an attractor (as are the opposite two projections), however it’s neither some extent nor a cycle. It’s an attractor
within the sense that ranging from quite a lot of completely different preliminary values, we find yourself in some sub-region of the state area, and we
don’t get to flee no extra. That is simpler to see when watching evolution over time, as on this animation:
Determine 7: How the Lorenz attractor traces out the well-known “butterfly” form.
Now, to plot the attractor in two dimensions, we threw away the third. However in “actual life,” we don’t normally have too a lot
info (though it might typically seem to be we had). We’d have a whole lot of measurements, however these don’t normally mirror
the precise state variables we’re involved in. In these circumstances, we might need to really add info.
Embeddings (as a non-DL time period), or: Undoing the projection
Assume that as a substitute of all three variables of the Lorenz system, we had measured only one: (x), the speed of convection. Typically
in nonlinear dynamics, the strategy of delay coordinate embedding (Sauer, Yorke, and Casdagli 1991) is used to boost a collection of univariate
measurements.
On this methodology – or household of strategies – the univariate collection is augmented by time-shifted copies of itself. There are two
choices to be made: What number of copies so as to add, and the way massive the delay must be. For instance, if we had a scalar collection,
1 2 3 4 5 6 7 8 9 10 11 ...a three-dimensional embedding with time delay 2 would appear like this:
1 3 5
2 4 6
3 5 7
4 6 8
5 7 9
6 8 10
7 9 11
...Of the 2 choices to be made – variety of shifted collection and time lag – the primary is a call on the dimensionality of
the reconstruction area. Varied theorems, comparable to Taken’s theorem,
point out bounds on the variety of dimensions required, offered the dimensionality of the true state area is thought – which,
in real-world functions, usually shouldn’t be the case.The second has been of little curiosity to mathematicians, however is necessary
in apply. In reality, Kantz and Schreiber (Kantz and Schreiber 2004) argue that in apply, it’s the product of each parameters that issues,
because it signifies the time span represented by an embedding vector.
How are these parameters chosen? Concerning reconstruction dimensionality, the reasoning goes that even in chaotic programs,
factors which might be shut in state area at time (t) ought to nonetheless be shut at time (t + Delta t), offered (Delta t) could be very
small. So say we have now two factors which might be shut, by some metric, when represented in two-dimensional area. However in three
dimensions, that’s, if we don’t “venture away” the third dimension, they’re much more distant. As illustrated in
(Gilpin 2020):
Determine 8: Within the two-dimensional projection on axes x and y, the crimson factors are shut neighbors. In 3d, nevertheless, they’re separate. Examine with the blue factors, which keep shut even in higher-dimensional area. Determine from Gilpin (2020).
If this occurs, then projecting down has eradicated some important info. In 2nd, the factors have been false neighbors. The
false nearest neighbors (FNN) statistic can be utilized to find out an satisfactory embedding measurement, like this:
For every level, take its closest neighbor in (m) dimensions, and compute the ratio of their distances in (m) and (m+1)
dimensions. If the ratio is bigger than some threshold (t), the neighbor was false. Sum the variety of false neighbors over all
factors. Do that for various (m) and (t), and examine the ensuing curves.
At this level, let’s look forward on the autoencoder strategy. The autoencoder will use that very same FNN statistic as a
regularizer, along with the same old autoencoder reconstruction loss. This may end in a brand new heuristic relating to embedding
dimensionality that includes fewer choices.
Going again to the traditional methodology for an instantaneous, the second parameter, the time lag, is much more troublesome to type out
(Kantz and Schreiber 2004). Often, mutual info is plotted for various delays after which, the primary delay the place it falls under some
threshold is chosen. We don’t additional elaborate on this query as it’s rendered out of date within the neural community strategy.
Which we’ll see now.
Studying the Lorenz attractor
Our code intently follows the structure, parameter settings, and information setup used within the reference
implementation William offered. The loss operate, particularly, has been ported
one-to-one.
The overall thought is the next. An autoencoder – for instance, an LSTM autoencoder as offered right here – is used to compress
the univariate time collection right into a latent illustration of some dimensionality, which is able to represent an higher certain on the
dimensionality of the discovered attractor. Along with imply squared error between enter and reconstructions, there might be a
second loss time period, making use of the FNN regularizer. This leads to the latent items being roughly ordered by significance, as
measured by their variance. It’s anticipated that someplace within the itemizing of variances, a pointy drop will seem. The items
earlier than the drop are then assumed to encode the attractor of the system in query.
On this setup, there may be nonetheless a option to be made: methods to weight the FNN loss. One would run coaching for various weights
(lambda) and search for the drop. Certainly, this might in precept be automated, however given the novelty of the strategy – the
paper was printed this yr – it is sensible to give attention to thorough evaluation first.
Knowledge era
We use the deSolve package deal to generate information from the Lorenz equations.
library(deSolve)
library(tidyverse)
parameters <- c(sigma = 10,
rho = 28,
beta = 8/3)
initial_state <-
c(x = -8.60632853,
y = -14.85273055,
z = 15.53352487)
lorenz <- operate(t, state, parameters) {
with(as.checklist(c(state, parameters)), {
dx <- sigma * (y - x)
dy <- x * (rho - z) - y
dz <- x * y - beta * z
checklist(c(dx, dy, dz))
})
}
instances <- seq(0, 500, size.out = 125000)
lorenz_ts <-
ode(
y = initial_state,
instances = instances,
func = lorenz,
parms = parameters,
methodology = "lsoda"
) %>% as_tibble()
lorenz_ts[1:10,]# A tibble: 10 x 4
time x y z
<dbl> <dbl> <dbl> <dbl>
1 0 -8.61 -14.9 15.5
2 0.00400 -8.86 -15.2 15.9
3 0.00800 -9.12 -15.6 16.3
4 0.0120 -9.38 -16.0 16.7
5 0.0160 -9.64 -16.3 17.1
6 0.0200 -9.91 -16.7 17.6
7 0.0240 -10.2 -17.0 18.1
8 0.0280 -10.5 -17.3 18.6
9 0.0320 -10.7 -17.7 19.1
10 0.0360 -11.0 -18.0 19.7We’ve already seen the attractor, or reasonably, its three two-dimensional projections, in determine 6 above. However now our situation is
completely different. We solely have entry to (x), a univariate time collection. Because the time interval used to numerically combine the
differential equations was reasonably tiny, we simply use each tenth remark.
Determine 9: Convection charges as a univariate time collection.
Preprocessing
The primary half of the collection is used for coaching. The information is scaled and remodeled into the three-dimensional type anticipated
by recurrent layers.
library(keras)
library(tfdatasets)
library(tfautograph)
library(reticulate)
library(purrr)
# scale observations
obs <- obs %>% mutate(
x = scale(x)
)
# generate timesteps
n <- nrow(obs)
n_timesteps <- 10
gen_timesteps <- operate(x, n_timesteps) {
do.name(rbind,
purrr::map(seq_along(x),
operate(i) {
begin <- i
finish <- i + n_timesteps - 1
out <- x[start:end]
out
})
) %>%
na.omit()
}
# practice with begin of time collection, take a look at with finish of time collection
x_train <- gen_timesteps(as.matrix(obs$x)[1:(n/2)], n_timesteps)
x_test <- gen_timesteps(as.matrix(obs$x)[(n/2):n], n_timesteps)
# add required dimension for options (we have now one)
dim(x_train) <- c(dim(x_train), 1)
dim(x_test) <- c(dim(x_test), 1)
# some batch measurement (worth not essential)
batch_size <- 100
# rework to datasets so we will use customized coaching
ds_train <- tensor_slices_dataset(x_train) %>%
dataset_batch(batch_size)
ds_test <- tensor_slices_dataset(x_test) %>%
dataset_batch(nrow(x_test))Autoencoder
With newer variations of TensorFlow (>= 2.0, actually if >= 2.2), autoencoder-like fashions are finest coded as customized fashions,
and skilled in an “autographed” loop.
The encoder is centered round a single LSTM layer, whose measurement determines the utmost dimensionality of the attractor. The
decoder then undoes the compression – once more, primarily utilizing a single LSTM.
# measurement of the latent code
n_latent <- 10L
n_features <- 1
encoder_model <- operate(n_timesteps,
n_features,
n_latent,
identify = NULL) {
keras_model_custom(identify = identify, operate(self) {
self$noise <- layer_gaussian_noise(stddev = 0.5)
self$lstm <- layer_lstm(
items = n_latent,
input_shape = c(n_timesteps, n_features),
return_sequences = FALSE
)
self$batchnorm <- layer_batch_normalization()
operate (x, masks = NULL) {
x %>%
self$noise() %>%
self$lstm() %>%
self$batchnorm()
}
})
}
decoder_model <- operate(n_timesteps,
n_features,
n_latent,
identify = NULL) {
keras_model_custom(identify = identify, operate(self) {
self$repeat_vector <- layer_repeat_vector(n = n_timesteps)
self$noise <- layer_gaussian_noise(stddev = 0.5)
self$lstm <- layer_lstm(
items = n_latent,
return_sequences = TRUE,
go_backwards = TRUE
)
self$batchnorm <- layer_batch_normalization()
self$elu <- layer_activation_elu()
self$time_distributed <- time_distributed(layer = layer_dense(items = n_features))
operate (x, masks = NULL) {
x %>%
self$repeat_vector() %>%
self$noise() %>%
self$lstm() %>%
self$batchnorm() %>%
self$elu() %>%
self$time_distributed()
}
})
}
encoder <- encoder_model(n_timesteps, n_features, n_latent)
decoder <- decoder_model(n_timesteps, n_features, n_latent)Loss
As already defined above, the loss operate we practice with is twofold. On the one hand, we examine the unique inputs with
the decoder outputs (the reconstruction), utilizing imply squared error:
mse_loss <- tf$keras$losses$MeanSquaredError(
discount = tf$keras$losses$Discount$SUM)As well as, we attempt to maintain the variety of false neighbors small, by way of the next regularizer.
loss_false_nn <- operate(x) {
# authentic values utilized in Kennel et al. (1992)
rtol <- 10
atol <- 2
k_frac <- 0.01
okay <- max(1, ground(k_frac * batch_size))
tri_mask <-
tf$linalg$band_part(
tf$ones(
form = c(n_latent, n_latent),
dtype = tf$float32
),
num_lower = -1L,
num_upper = 0L
)
batch_masked <- tf$multiply(
tri_mask[, tf$newaxis,], x[tf$newaxis, reticulate::py_ellipsis()]
)
x_squared <- tf$reduce_sum(
batch_masked * batch_masked,
axis = 2L,
keepdims = TRUE
)
pdist_vector <- x_squared +
tf$transpose(
x_squared, perm = c(0L, 2L, 1L)
) -
2 * tf$matmul(
batch_masked,
tf$transpose(batch_masked, perm = c(0L, 2L, 1L))
)
all_dists <- pdist_vector
all_ra <-
tf$sqrt((1 / (
batch_size * tf$vary(1, 1 + n_latent, dtype = tf$float32)
)) *
tf$reduce_sum(tf$sq.(
batch_masked - tf$reduce_mean(batch_masked, axis = 1L, keepdims = TRUE)
), axis = c(1L, 2L)))
all_dists <- tf$clip_by_value(all_dists, 1e-14, tf$reduce_max(all_dists))
top_k <- tf$math$top_k(-all_dists, tf$solid(okay + 1, tf$int32))
top_indices <- top_k[[1]]
neighbor_dists_d <- tf$collect(all_dists, top_indices, batch_dims = -1L)
neighbor_new_dists <- tf$collect(
all_dists[2:-1, , ],
top_indices[1:-2, , ],
batch_dims = -1L
)
# Eq. 4 of Kennel et al. (1992)
scaled_dist <- tf$sqrt((
tf$sq.(neighbor_new_dists) -
tf$sq.(neighbor_dists_d[1:-2, , ])) /
tf$sq.(neighbor_dists_d[1:-2, , ])
)
# Kennel situation #1
is_false_change <- (scaled_dist > rtol)
# Kennel situation #2
is_large_jump <-
(neighbor_new_dists > atol * all_ra[1:-2, tf$newaxis, tf$newaxis])
is_false_neighbor <-
tf$math$logical_or(is_false_change, is_large_jump)
total_false_neighbors <-
tf$solid(is_false_neighbor, tf$int32)[reticulate::py_ellipsis(), 2:(k + 2)]
reg_weights <- 1 -
tf$reduce_mean(tf$solid(total_false_neighbors, tf$float32), axis = c(1L, 2L))
reg_weights <- tf$pad(reg_weights, checklist(checklist(1L, 0L)))
activations_batch_averaged <-
tf$sqrt(tf$reduce_mean(tf$sq.(x), axis = 0L))
loss <- tf$reduce_sum(tf$multiply(reg_weights, activations_batch_averaged))
loss
}MSE and FNN are added , with FNN loss weighted based on the important hyperparameter of this mannequin:
This worth was experimentally chosen because the one finest conforming to our look-for-the-highest-drop heuristic.
Mannequin coaching
The coaching loop intently follows the aforementioned recipe on methods to
practice with customized fashions and tfautograph.
train_loss <- tf$keras$metrics$Imply(identify='train_loss')
train_fnn <- tf$keras$metrics$Imply(identify='train_fnn')
train_mse <- tf$keras$metrics$Imply(identify='train_mse')
train_step <- operate(batch) {
with (tf$GradientTape(persistent = TRUE) %as% tape, {
code <- encoder(batch)
reconstructed <- decoder(code)
l_mse <- mse_loss(batch, reconstructed)
l_fnn <- loss_false_nn(code)
loss <- l_mse + fnn_weight * l_fnn
})
encoder_gradients <- tape$gradient(loss, encoder$trainable_variables)
decoder_gradients <- tape$gradient(loss, decoder$trainable_variables)
optimizer$apply_gradients(
purrr::transpose(checklist(encoder_gradients, encoder$trainable_variables))
)
optimizer$apply_gradients(
purrr::transpose(checklist(decoder_gradients, decoder$trainable_variables))
)
train_loss(loss)
train_mse(l_mse)
train_fnn(l_fnn)
}
training_loop <- tf_function(autograph(operate(ds_train) {
for (batch in ds_train) {
train_step(batch)
}
tf$print("Loss: ", train_loss$consequence())
tf$print("MSE: ", train_mse$consequence())
tf$print("FNN loss: ", train_fnn$consequence())
train_loss$reset_states()
train_mse$reset_states()
train_fnn$reset_states()
}))
optimizer <- optimizer_adam(lr = 1e-3)
for (epoch in 1:200) {
cat("Epoch: ", epoch, " -----------n")
training_loop(ds_train)
}After 200 epochs, total loss is at 2.67, with the MSE part at 1.8 and FNN at 0.09.
Acquiring the attractor from the take a look at set
We use the take a look at set to examine the latent code:
# A tibble: 6,242 x 10
V1 V2 V3 V4 V5 V6 V7 V8 V9 V10
<dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 0.439 0.401 -0.000614 -0.0258 -0.00176 -0.0000276 0.000276 0.00677 -0.0239 0.00906
2 0.415 0.504 0.0000481 -0.0279 -0.00435 -0.0000970 0.000921 0.00509 -0.0214 0.00921
3 0.389 0.619 0.000848 -0.0240 -0.00661 -0.000171 0.00106 0.00454 -0.0150 0.00794
4 0.363 0.729 0.00137 -0.0143 -0.00652 -0.000244 0.000523 0.00450 -0.00594 0.00476
5 0.335 0.809 0.00128 -0.000450 -0.00338 -0.000307 -0.000561 0.00407 0.00394 -0.000127
6 0.304 0.828 0.000631 0.0126 0.000889 -0.000351 -0.00167 0.00250 0.0115 -0.00487
7 0.274 0.769 -0.000202 0.0195 0.00403 -0.000367 -0.00220 -0.000308 0.0145 -0.00726
8 0.246 0.657 -0.000865 0.0196 0.00558 -0.000359 -0.00208 -0.00376 0.0134 -0.00709
9 0.224 0.535 -0.00121 0.0162 0.00608 -0.000335 -0.00169 -0.00697 0.0106 -0.00576
10 0.211 0.434 -0.00129 0.0129 0.00606 -0.000306 -0.00134 -0.00927 0.00820 -0.00447
# … with 6,232 extra rowsBecause of the FNN regularizer, the latent code items must be ordered roughly by lowering variance, with a pointy drop
showing some place (if the FNN weight has been chosen adequately).
For an fnn_weight of 10, we do see a drop after the primary two items:
predicted %>% summarise_all(var)# A tibble: 1 x 10
V1 V2 V3 V4 V5 V6 V7 V8 V9 V10
<dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 0.0739 0.0582 1.12e-6 3.13e-4 1.43e-5 1.52e-8 1.35e-6 1.86e-4 1.67e-4 4.39e-5So the mannequin signifies that the Lorenz attractor will be represented in two dimensions. If we nonetheless need to plot the
full (reconstructed) state area of three dimensions, we should always reorder the remaining variables by magnitude of
variance. Right here, this leads to three projections of the set V1, V2 and V4:
Determine 10: Attractors as predicted from the latent code (take a look at set). The three highest-variance variables have been chosen.
Wrapping up (for this time)
At this level, we’ve seen methods to reconstruct the Lorenz attractor from information we didn’t practice on (the take a look at set), utilizing an
autoencoder regularized by a customized false nearest neighbors loss. It is very important stress that at no level was the community
offered with the anticipated resolution (attractor) – coaching was purely unsupervised.
It is a fascinating consequence. In fact, pondering virtually, the subsequent step is to acquire predictions on heldout information. Given
how lengthy this textual content has change into already, we reserve that for a follow-up put up. And once more after all, we’re excited about different
datasets, particularly ones the place the true state area shouldn’t be identified beforehand. What about measurement noise? What about
datasets that aren’t fully deterministic? There’s a lot to discover, keep tuned – and as at all times, thanks for
studying!
Kantz, Holger, and Thomas Schreiber. 2004. Nonlinear Time Collection Evaluation. Cambridge College Press.
Strang, Gilbert. 2019. Linear Algebra and Studying from Knowledge. Wellesley Cambridge Press.
Strogatz, Steven. 2015. Nonlinear Dynamics and Chaos: With Functions to Physics, Biology, Chemistry, and Engineering. Westview Press.
