if( !require('phaseR') ){
    install.packages('phaseR')
}
library(phaseR)

my_yprime = function(t, y, parameters) {
    xt = y[1]
    yt = y[2]
    dy = rep(NA, length(y))
    dy[1] = -xt
    dy[2] = 2*yt
    list(dy)
}

diff_eq_params = list()

#postscript('../../WriteUp/Graphics/Chapter9/chap_9_sect_2_prob_2_plot.eps', onefile=FALSE, horizontal=FALSE)

t.end = 2.0
L = 5
flowField(my_yprime, x.lim = c(-L, +L), y.lim = c(-L, +L), parameters = diff_eq_params, points = 21, add = FALSE, xlab='x', ylab='y')
trajectory(my_yprime, y0 = c(4, 2), t.end = t.end, parameters = diff_eq_params, col='black', pch=19)
trajectory(my_yprime, y0 = c(4, 0), t.end = t.end, parameters = diff_eq_params, col='blue', pch=19)
grid()

#dev.off()