def cobweb(a_function, start, mask = 0, iterations = 20, xmin = 0, xmax = 1):
'''
Returns a graphics object of a plot of the function and a cobweb trajectory starting from the value start.
INPUT:
a_function: a function of one variable
start: the starting value of the iteration
mask: (optional) the number of initial iterates to ignore
iterations: (optional) the number of iterations to draw, following the masked iterations
xmin: (optional) the lower end of the plotted interval
xmax: (optional) the upper end of the plotted interval
EXAMPLES:
sage: f = lambda x: 3.9*x*(1-x)
sage: show(cobweb(f,.01,iterations=200), xmin = 0, xmax = 1, ymin=0)
'''
basic_plot = plot(a_function, xmin = xmin, xmax = xmax)
id_plot = plot(lambda x: x, xmin = xmin, xmax = xmax)
iter_list = []
current = start
for i in range(mask):
current = a_function(current)
for i in range(iterations):
iter_list.append([current,a_function(current)])
current = a_function(current)
iter_list.append([current,current])
cobweb = line(iter_list, rgbcolor = (1,0,0))
return basic_plot + id_plot + cobweb
var('x')
@interact
def cobwebber(f_text = input_box(default = "3.8*x*(1-x)",label = "function", type=str), start_val = slider(0,1,.01,.5,label = 'start value'), its = slider([2^i for i in range(0,12)],default = 16, label="iterations")):
def f(x):
return eval(f_text)
show(cobweb(f, start_val, iterations = its))
