A258091 Smallest prime factor of 1+78557*2^n, cf. A258073.

5, 3, 73, 3, 5, 3, 7, 3, 5, 3, 13, 3, 5, 3, 19, 3, 5, 3, 7, 3, 5, 3, 13, 3, 5, 3, 37, 3, 5, 3, 7, 3, 5, 3, 13, 3, 5, 3, 71, 3, 5, 3, 7, 3, 5, 3, 13, 3, 5, 3, 19, 3, 5, 3, 7, 3, 5, 3, 13, 3, 5, 3, 37, 3, 5, 3, 7, 3, 5, 3, 13, 3, 5, 3, 73, 3, 5, 3, 7, 3, 5, 3

Offset

1,1

Comments

a(n) = A020639(A258073(a(n)));

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

Cf. A020639, A258073, A258095.

Sequence in context: A007299 A257935 A109254 * A370551 A350213 A255599

Adjacent sequences: A258088 A258089 A258090 * A258092 A258093 A258094

Keyword

nonn,easy

Author

Reinhard Zumkeller, May 19 2015

Status

approved