

A281818


a(n) is the least number k such that sigma(n) + sigma(k) = sigma(abs(nk)), or 1 if such a number does not exist.


1




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(51).


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(nk)) 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



STATUS

approved



