pro prism_funct, x, a, f, isel, pder

f = a(1) + a(2)/(x-a(0)) + a(3)/(x-a(0))^2  + a(4)/(x-a(0))^3 + $
			a(5)/(x-a(0))^4

if n_params() ge 5 then begin
    pder = fltarr(n_elements(x),n_elements(isel))
    for i=0,n_elements(isel)-1 do begin
	    case isel(i) of
		0:    pder(*,0) = a(2)/(x-a(0))^2 + 2*a(3)/(x-a(0))^3
		1:    pder(*,1) = 1.0
		2:    pder(*,2) = 1.0/(x-a(0))
		3:    pder(*,3) = 1.0/(x-a(0))^2
		4:    pder(*,4) = 1.0/(x-a(0))^3
		5:    pder(*,5) = 1.0/(x-a(0))^4
	    end
    end
endif
return
end