##
How to solve the Fourier Series problem

install.packages("fourierin", dependencies = FALSE)

> library("dplyr", lib.loc="~/R/win-library/3.6")

> library("ggplot2", lib.loc="~/R/win-library/3.6")

install.packages("purrrlyr", dependencies = FALSE)

install.packages("purrr", dependencies = FALSE)

library("dplyr", lib.loc="~/R/win-library/3.6")

> library("lattice", lib.loc="C:/Program Files/R/R-3.6.1/library")

> df <- 5

> cf <- function(t) (1 - 2i*t)^(-df/2)

> dens <- function(x) dchisq(x, df)

> ## Set resolutions

> resolutions <- 2^(6:8)

> ## Compute integral given the resoltion

> recover_f <- function(resol){

+ ## Get grid and density values

+ out <- fourierin(f = cf, lower_int = -10, upper_int = 10,

+ lower_eval = -3, upper_eval = 20,

+ const_adj = -1, freq_adj = -1,

+ resolution = resol)

+ ## Return in dataframe format

+ out %>%

+ as_data_frame() %>%

+ transmute(

+ x = w,

+ values = Re(values),

+ resolution = resol)

+ }

> vals <- map_df(resolutions, recover_f)

>

> true <- data_frame(x = seq(min(vals$x), max(vals$x), length = 150),

+ values = dens(x))

Warning message:

`data_frame()` is deprecated, use `tibble()`.

This warning is displayed once per session.

>

> univ_plot <-

+ vals %>%

+ mutate(resolution = as.character(resolution),

+ resolution = gsub("64", "064", resolution)) %>%

+ ggplot(aes(x, values)) +

+ geom_line(aes(color = resolution)) +

+ geom_line(data = true, aes(color = "true values"))

> univ_plot

> set.seed(666)

> new_grid <- rchisq(n = 3, df = df)

> resolutions <- 2^(6:9)

> fourierin(f = cf, lower_int = -10, upper_int = 10,

+ eval_grid = new_grid,

+ const_adj = -1, freq_adj = -1,

+ resolution = 128) %>%

+ c () %>% Re() %>%

+ data_frame(x = new_grid, fx = .)

# A tibble: 3 x 2

x fx

<dbl> <dbl>

1 6.41 0.0874

2 11.7 0.0152

3 3.06 0.154

> approximated_fx <- function (resol) {

+ fourierin(f = cf, lower_int = -10, upper_int = 12,

+ eval_grid = new_grid,

+ const_adj = -1, freq_adj = -1,

+ resolution = resol) %>%

+ c() %>% Re() %>%

+ {data_frame(x = new_grid,

+ fx = dens(new_grid),

+ diffs = abs(. - fx),

+ resolution = resol)}

+ }

>

> tab <-

+ map_df(resolutions, approximated_fx) %>%

+ arrange(x) %>%

+ mutate(diffs = round(diffs, 7)) %>%

+ rename('f(x)' = fx,

+ 'absolute difference' = diffs)

> tab

# A tibble: 12 x 4

x `f(x)` `absolute differe~ resolution

<dbl> <dbl> <dbl> <dbl>

1 3.06 0.154 0.000207 64

2 3.06 0.154 0.0000476 128

3 3.06 0.154 0.0000472 256

4 3.06 0.154 0.0000471 512

5 6.41 0.0874 0.000078 64

6 6.41 0.0874 0.0000155 128

7 6.41 0.0874 0.0000148 256

8 6.41 0.0874 0.0000147 512

9 11.7 0.0152 0.0000097 64

10 11.7 0.0152 0.0000025 128

11 11.7 0.0152 0.0000022 256

12 11.7 0.0152 0.0000022 512

> library("dplyr", lib.loc="~/R/win-library/3.6")

> library("lattice", lib.loc="C:/Program Files/R/R-3.6.1/library")

mu <- c(-1, 1)

> sig <- matrix(c(3, -1, -1, 2), 2, 2)

> f <- function(x) {

+ ## Auxiliar values

+ d <- ncol(x)

+ z <- sweep(x, 2, mu, "-")

+ ## Get numerator and denominator of normal density

+ num <- exp(-0.5*rowSums(z * (z %*% solve(sig))))

+ denom <- sqrt((2*pi)^d*det(sig))

+ return(num/denom)

+ }

>

> ## Characteristic function, s is n x d

> phi <- function (s) {

+ complex(modulus = exp(-0.5*rowSums(s*(s %*% sig))),

+ argument = s %*% mu)

+ }

> ## Evaluate characteristic function for a given resolution.

> eval <- fourierin(f,

+ lower_int = c(-8, -6), upper_int = c(6, 8),

+ lower_eval = c(-4, -4), upper_eval = c(4, 4),

+ const_adj = 1, freq_adj = 1,

+ resolution = 2*c(64, 64),

+ use_fft = T)

> ## Evaluate true and approximated values of Fourier integral

> dat <- eval %>%

+ with(crossing(y = w2, x = w1) %>%

+ mutate(approximated = c(values))) %>%

+ mutate(true = phi(matrix(c(x, y), ncol = 2)),

+ difference = approximated - true) %>%

+ gather(value, z, -x, -y) %>%

+ mutate(real = Re(z), imaginary = Im(z)) %>%

+ select(-z) %>%

+ gather(part, z, -x, -y, -value)

Error in crossing(y = w2, x = w1) : could not find function "crossing"

> wireframe(z ~ x*y | value*part, data = dat,

+ scales =

+ list(arrows=FALSE, cex= 0.45,

+ col = "black", font = 3, tck = 1),

+ screen = list(z = 90, x = -74),

+ colorkey = FALSE,

+ shade=TRUE,

+ light.source= c(0,10,10),

+ shade.colors = function(irr, ref,

+ height, w = 0.4)

+ grey(w*irr + (1 - w)*(1 - (1 - ref)^0.4)),

+ aspect = c(1, 0.65))

Error in wireframe.formula(z ~ x * y | value * part, data = dat, scales = list(arrows = FALSE, :

object 'dat' not found

>

> mu <- c(-1, 1)

> sig <- matrix(c(3, -1, -1, 2), 2, 2)

> f <- function(x) {

+ ##-Auxiliar values

+ d <- ncol(x)

+ z <- sweep(x, 2, mu, "-")

+ ##-Get numerator and denominator of normal density

+ num <- exp(-0.5*rowSums(z * (z %*% solve(sig))))

+ denom <- sqrt((2*pi)^d*det(sig))

+

+ return(num/denom)

+ }

> phi <- function(s) {

+ complex(modulus = exp(- 0.5*rowSums(s*(s %*% sig))),

+ argument = s %*% mu)

+ }

>

> eval <- fourierin_2d(f, lower_int = c(-8, -6), upper_int = c(6, 8),

+ lower_eval = c(-4, -4), upper_eval = c(4, 4),

+ const_adj = 1, freq_adj = 1,

+ resolution = c(128, 128))

>

> t1 <- eval$w1

> t2 <- eval$w2

> t <- as.matrix(expand.grid(t1 = t1, t2 = t2))

> approx <- eval$values

> true <- matrix(phi(t), 128, 128) # Compute true values

>

> i <- 65

> plot(t2, Re(approx[i, ]), type = "l", col = 2,

+ ylab = "",

+ xlab = expression(t[2]),

+ main = expression(paste("Real part section at ",

+ t[1], "= 0")))

> lines(t2, Re(true[i, ]), col = 3)

> legend("topleft", legend = c("true", "approximation"),

+ col = 3:2, lwd = 1)

> plot(t1, Im(approx[, i]), type = "l", col = 2,

+ ylab = "",

+ xlab = expression(t[1]),

+ main = expression(paste("Imaginary part section at ",

+ t[2], "= 0")))

> lines(t1, Im(true[, i]), col = 3)

> legend("topleft", legend = c("true", "approximation"),

+ col = 3:2, lwd = 1)

>