epicours/Algo/Courses/Chapter 7 - X (exercises).md

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```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;;