```Ocaml
# let sum n = 
	if n < 0 then
		invalid_arg "n<0"
	else
		let rec sumrc n  =
			if n=0 then
				0
			else
				n + sum(n-1)
		in sumrc n;;
val sum : int -> int = <fun>

(*Correction*)
let sigma f n = 
	if n<0 then
		invalid_arg "n<0"
	else
		let rec sig = function
			| 0 -> f0
			| n -> fn + sig (n-1)
		in sig n;;