|
%I #12 Sep 08 2022 08:46:15
%S 2,3,7,1261,11242771
%N a(n) is the smallest number k such that phi(k) = n*phi(k-1).
%e a(3) = 7 because 7 is the smallest number k such that phi(k) = n*phi(k-1); phi(7) = 6 =3*phi(6) = 3*2.
%o (Magma) a:=func<n|exists(r){k:k in[2..10^8] | EulerPhi(k) eq n*EulerPhi(k-1)}select r else 0>; [a(n):n in[1..5]]
%o (PARI) a(n) = my(k=2, epk=1, enk); while ((enk=eulerphi(k)) != n*epk, epk = enk; k++); k; \\ _Michel Marcus_, Feb 20 2020
%Y Sequences of numbers n such that phi(n) = k*phi(n-1) for k = 1, 2, 3, 4, 5: {A001274 + 1} for k=1; A171271 = {A050472 + 1} for k=2; A266268 = {A067143 + 1} for k=3; A268126 = {A172314 + 1} for k=4; {A201253 + 1} for k=5.
%Y Cf. A000010 (phi(n)), A266269 (the smallest numbers k such that phi(k) >= n*phi(k-1)).
%K nonn,more
%O 1,1
%A _Jaroslav Krizek_, Jan 26 2016
|
