function f = fsin(x,n,c)
%
%  Find nth derivative of sin(c*x) at x
%  n=0  f=sin(c*x)
%  n=1  f=c*cos(c*x)
%  n=2  f=-c^2*sin(c*x)
%  n=3  f=-c^3*cos(c*x)
%  and the cycle repeats for n>3
%
switch mod(n,4)
    case 0  %% n = 0, 4, etc.
        f = c^n*sin(c*x);
    case 1
        f = c^n*cos(c*x);
    case 2
        f = -c^n*sin(c*x);
    case 3
        f = -c^n*cos(c*x);
end