OFFSET
1,4
COMMENTS
For any positive integer n, if k = a(n) + n*m*A007736(n) and m >= 0 then 5^k + k is divisible by n.
LINKS
Brazil National Olympiad, 2005, Problem 6
FORMULA
a(5^m) = 5^m for m >= 0.
PROG
(PARI) a(n) = for(k=1, oo, if(Mod(5, n)^k==-k, return(k)));
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jinyuan Wang, Apr 14 2020
STATUS
approved