A216394 - OEIS

%I #27 Jan 07 2025 15:16:22

%S 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 4208,4156

%N 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 a(n) = # { k in A115921 | 2^(n-1) < k < 2^n }. - _M. F. Hasler_, Feb 24 2014

%e 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 (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 (Python)

%o from sympy import totient

%o def A216394(n):

%o     if n == 1:

%o         return 1

%o     c = 0

%o     for i in range(2**(n-1)+1, 2**n):

%o         s1, s2 = sorted(str(i)), sorted(str(totient(i)))

%o         if len(s1) == len(s2) and s1 == s2:

%o             c += 1

%o     return c # _Chai Wah Wu_, Jul 23 2015

%Y Cf. A115921, A000010.

%K nonn,base,more

%O 1,9

%A _V. Raman_, Sep 06 2012