OFFSET
1,1
COMMENTS
a(8) > 10^14 unless a(8) = -1. (It's relatively easy to test because k needs to be a square or twice a square.) - Charles R Greathouse IV , Feb 03 2017
LINKS
EXAMPLE
a(1) = 5 because 5 is the least number such that sigma(1) + sigma(5) = 1 + 6 = 7 = sigma(5-1).
MAPLE
with(numtheory): P:= proc(q) local k, n; for n from 1 to q do for k from 1 to q do
if sigma(n)+sigma(k)=sigma(abs(n-k)) then print(k); break; fi;
od; if k=q+1 then print(-1); fi; od; end: P(10^6);
MATHEMATICA
Table[SelectFirst[Range[10^6], DivisorSigma[1, n] + DivisorSigma[1, #] == DivisorSigma[1, Abs[n - #]] &] /. k_ /; MissingQ@ k -> -1, {n, 58}] (* Michael De Vlieger, Feb 01 2017, Version 10.2 *)
CROSSREFS
KEYWORD
sign,more
AUTHOR
Paolo P. Lava, Jan 31 2017
STATUS
approved