OFFSET
0,4
COMMENTS
If n is a power of 2 then a(n)=1. All other positive n have a(n)>1. a(n)=2 precisely in cases where some multiple of n is a factor of 2^q+1 for some q.
LINKS
T. D. Noe, Table of n, a(n) for n = 0..10000
Trevor Clokie et al., Computational Aspects of Sturdy and Flimsy Numbers, arxiv preprint arXiv:2002.02731 [cs.DS], February 7 2020.
FORMULA
a(2^k-1) = k. - Thomas Dybdahl Ahle, May 01 2013
EXAMPLE
a(n)=2 for n=53, 59, 61, 67, 81, 97 and 101 because n divides 2^k+1 for k=26, 29, 30, 33, 27, 24 and 50, respectively. - T. D. Noe, Jul 22 2008
PROG
(PARI) a(n)=if(!n, return(0)); n>>=valuation(n, 2); my(o=znorder(Mod(2, n)), v1=Set(powers(Mod(2, n), o)), v=v1, s=1); while(!setsearch(v, Mod(0, n)), v=setbinop((x, y)->x+y, v, v1); s++); s \\ Charles R Greathouse IV, Dec 07 2016
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Sean A. Irvine, Sep 02 2003
EXTENSIONS
More terms from Robert G. Wilson v, Feb 21 2005
Corrected by T. D. Noe, Jul 22 2008
An incorrect Mathematica program was deleted Aug 01 2008
STATUS
approved