OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..500
Paolo P. Lava, First 250 terms with the ratio sigma(R(k))/sigma(k)
EXAMPLE
a(1) = 15 because sigma(51) / sigma(15) = 72 / 24 = 3;
a(2) = 16 because sigma(61) / sigma(16) = 62 / 31 = 2;
a(3) = 17 because sigma(71) / sigma(17) = 72 / 18 = 4.
MAPLE
with(numtheory): T:=proc(w) local x, y, z; x:=w; y:=0;
for z from 1 to ilog10(x)+1 do y:=10*y+(x mod 10); x:=trunc(x/10); od; y; end:
P:=proc(q) local n; for n from 1 to q do
if n<>T(n) then if type(sigma(T(n))/sigma(n), integer) then print(n); fi; fi; od; end: P(10^6);
MATHEMATICA
Select[Range[6000], !PalindromeQ[#]&&Mod[DivisorSigma[1, IntegerReverse[#]], DivisorSigma[ 1, #]] ==0&] (* Harvey P. Dale, Dec 19 2023 *)
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Paolo P. Lava, Feb 01 2017
STATUS
approved