# function returns point(s) on a straight line joining the element end points: 
# Ao * PHIo(X) + A1 * PHI1(X) at point(s) X

def points(x,x0,x1,a0,a1):     # function to return the line or a point within an element
    # x is a point or list of points, x0,x1, are the x value edges of the element, 
    # a0,a1 are the 'y' values of the 'f' function at the element edges
    temp = (x - x0)/(x1 - x0)    # note that x can be a list of numbers
    return a0*(1-temp) + a1*temp # can return a list, if x is a list