cp's OEIS Frontend

A216394 Number of values of k for which phi(k) is a permutation of decimal digits of k, for 2^(n-1) < k < 2^n.

%I A216394 #27 Jan 07 2025 15:16:22
%S A216394 1,0,0,0,1,1,0,0,2,0,0,3,11,2,13,21,26,49,91,186,108,335,937,500,1681,
%T A216394 4208,4156
%N A216394 Number of values of k for which phi(k) is a permutation of decimal digits of k, for 2^(n-1) < k < 2^n.
%F A216394 a(n) = # { k in A115921 | 2^(n-1) < k < 2^n }. - _M. F. Hasler_, Feb 24 2014
%e A216394 a(14) = 2 because the values of k satisfying the condition for 2^13 < k < 2^14 are {8541, 8982}. - _V. Raman_, Feb 18 2014
%o A216394 (PARI) a(n)=sum(k=2^(n-1), 2^n, vecsort(digits(k)) == vecsort(digits(eulerphi(k)))) \\ _V. Raman_, Feb 18 2014, based on edits by _M. F. Hasler_
%o A216394 (Python)
%o A216394 from sympy import totient
%o A216394 def A216394(n):
%o A216394     if n == 1:
%o A216394         return 1
%o A216394     c = 0
%o A216394     for i in range(2**(n-1)+1, 2**n):
%o A216394         s1, s2 = sorted(str(i)), sorted(str(totient(i)))
%o A216394         if len(s1) == len(s2) and s1 == s2:
%o A216394             c += 1
%o A216394     return c # _Chai Wah Wu_, Jul 23 2015
%Y A216394 Cf. A115921, A000010.
%K A216394 nonn,base,more
%O A216394 1,9
%A A216394 _V. Raman_, Sep 06 2012