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

Author

Reinhard Zumkeller, May 19 2015

Keywords

• nonn
• easy

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

Crossrefs

Cf. A020639, A258073, A258095.

Programs

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