OFFSET
1,121
COMMENTS
If n is square then sigma_2(n) is divisible by neither 2 nor 5. The product of residues is not always one. E.g., sigma_2(121) = 14673; mod 2 and mod 5 gives 1 and 3 residues. a(n)=3 for n=121, 361, 484, 841, 961 etc..
a(n)=4 for n=43681, 101761, 116281, 174724, 203401, 303601, 346921, ... - R. J. Mathar, Apr 02 2011
REFERENCES
S. Wolfram, A New Kind of Science, Wolfram Media, 2002; p. 55.
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..101761 (terms 1..10000 from Charles R Greathouse IV)
Robert Price, Comments on A065803 concerning Elementary Cellular Automata, Jan 29 2016
Eric Weisstein's World of Mathematics, Elementary Cellular Automaton
S. Wolfram, A New Kind of Science
MAPLE
A001157 := proc(n) numtheory[sigma][2](n) ; end proc:
MATHEMATICA
Array[Mod[#, 2] Mod[#, 5] &@ DivisorSigma[2, #] &, 121] (* Michael De Vlieger, Jan 19 2020 *)
PROG
(PARI) a(n)=if(issquare(n), sigma(n, 2)%5, 0) \\ Charles R Greathouse IV, Nov 19 2014
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Labos Elemer, Nov 21 2001
EXTENSIONS
Data section extended up to a(121) by Antti Karttunen, Jan 18 2020
STATUS
approved