cp's OEIS Frontend

A281880 Non-palindromic numbers k such that phi(k) | phi(R(k)), where R(k) is the digits reversal of k.

%I A281880 #19 Feb 06 2017 02:09:28
%S A281880 12,15,18,19,36,37,56,124,126,132,165,168,178,189,190,192,198,199,219,
%T A281880 234,238,298,308,348,387,396,418,427,429,468,506,518,724,756,924,1004,
%U A281880 1066,1078,1089,1094,1107,1143,1209,1212,1314,1332,1358,1364,1386,1445,1452
%N A281880 Non-palindromic numbers k such that phi(k) | phi(R(k)), where R(k) is the digits reversal of k.
%H A281880 Robert Israel, <a href="/A281880/b281880.txt">Table of n, a(n) for n = 1..1500</a>
%H A281880 Paolo P. Lava, <a href="/A281880/a281880.txt">First 250 terms with the ratio phi(R(k))/phi(k)</a>
%e A281880 a(1) = 12 because phi(21) / phi(12) = 12 / 4 = 3;
%e A281880 a(2) = 15 because phi(51) / phi(15) = 32 / 8 = 4;
%e A281880 a(3) = 18 because phi(81) / phi(18) = 54 / 6 = 9.
%p A281880 with(numtheory): T:=proc(w) local x, y, z; x:=w; y:=0;
%p A281880 for z from 1 to ilog10(x)+1 do y:=10*y+(x mod 10); x:=trunc(x/10); od; y; end:
%p A281880 P:=proc(q) local n; for n from 1 to q do
%p A281880 if n<>T(n) then if type(phi(T(n))/phi(n),integer) then print(n); fi; fi; od; end: P(10^6);
%t A281880 Select[Range@ 1500, Function[k, And[Reverse@ # != #, Divisible[EulerPhi[FromDigits@ Reverse@ #], EulerPhi@ k]] &@ IntegerDigits@ k]] (* _Michael De Vlieger_, Feb 04 2017 *)
%Y A281880 Cf. A000010, A004086, A097647, A281879.
%K A281880 nonn,easy,base
%O A281880 1,1
%A A281880 _Paolo P. Lava_, Feb 01 2017