cp's OEIS Frontend

A254320 Numbers k such that the reversal of phi(k) is sigma(k)-k.

%I A254320 #28 Sep 07 2024 03:21:26
%S A254320 2,11,101,735,7665,11505,16459,64578,378871,541033,3440409,5639353,
%T A254320 5230000213,5762782573,5828558173,8130408803,8275586723,9738107377,
%U A254320 11263073973,37057275961,38914628453,58285958173,231243884637,350649946051,380047486211,516420024613,547083380743,576216622573
%N A254320 Numbers k such that the reversal of phi(k) is sigma(k)-k.
%e A254320 sigma(2) - 2 = 1; rev(1) = 1 = phi(2).
%e A254320 sigma(735) - 735 = 633; rev(633) = 336 = phi(735).
%p A254320 with(numtheory):T:=proc(w) local x,y,z; x:=w; y:=0;
%p A254320 for z from 1 to ilog10(x)+1 do y:=10*y+(x mod 10); x:=trunc(x/10); od; y; end:
%p A254320 P:=proc(q) local n; for n from 1 to q do
%p A254320 if T(phi(n))=sigma(n)-n then print(n); fi; od; end: P(10^7);
%t A254320 Select[Range[564*10^4],IntegerReverse[EulerPhi[#]]==DivisorSigma[1,#]-#&] (* The program generates the first 12 terms of the sequence. *) (* _Harvey P. Dale_, Jul 03 2024 *)
%o A254320 (PARI) rev(n) = subst(Polrev(digits(n)), x, 10);
%o A254320 isok(n) = (sigma(n)-n) == rev(eulerphi(n)); \\ _Michel Marcus_, Jan 29 2015
%Y A254320 Cf. A000010, A000203, A001065, A069225, A247080.
%K A254320 nonn,base
%O A254320 1,1
%A A254320 _Paolo P. Lava_, Jan 28 2015
%E A254320 a(12) from _Michel Marcus_, Jan 29 2015
%E A254320 a(13)-a(28) from _Giovanni Resta_, May 08 2015