OFFSET
1,1
COMMENTS
a(n) <= 73; see also A258095.
Periodic, a(n) = a(n + 840420) for all n (and 840420 is minimal with this property). The only values that occur, are {3, 5, 7, 13, 19, 37, 73} union {47, 59, 71}. - Jeppe Stig Nielsen, Jul 19 2020
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Wikipedia, Sierpinski Number
PROG
(Haskell) a258091 = a020639 . a258073
(PARI) a(n)=forprime(p=2, , 78557*Mod(2, p)^n+1==0 && return(p)) \\ Jeppe Stig Nielsen, Jul 19 2020
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, May 19 2015
STATUS
approved