OFFSET
1,3
COMMENTS
If 2n-1 is prime, a(n) = (2n-3)!.
a(n) == 1 (mod 2n-1).
LINKS
Robert Israel, Table of n, a(n) for n = 1..228
Mathematics StackExchange, Product of x with x and x+1 relatively prime to n.
EXAMPLE
For n=5, the allowed k are 1, 4, 7, so a(5) = 1*4*7=28.
MAPLE
seq(convert(select(t -> igcd(t*(t+1), m)=1, [$1..m-1]), `*`), m = 1..100, 2);
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert Israel, Jul 27 2017
STATUS
approved