cp's OEIS Frontend

A216395 Number of values of k for which sigma(k) is a permutation of decimal digits of k, for 2^(n-1) < k < 2^n.

%I A216395 #28 Jan 07 2025 15:16:27
%S A216395 1,0,0,0,0,0,1,0,3,2,0,6,3,5,14,22,26,60,64,71,179,333,274,751,1653,
%T A216395 1726,3032
%N A216395 Number of values of k for which sigma(k) is a permutation of decimal digits of k, for 2^(n-1) < k < 2^n.
%F A216395 a(n) = # { k in A115920 | 2^(n-1) < k < 2^n }. - _M. F. Hasler_, Feb 24 2014
%e A216395 a(12) = 6 because the values of k satisfying the condition for 2^11 < k < 2^12 are {2391, 2556, 2931, 3409, 3678, 3679}. - _V. Raman_, Feb 19 2014
%o A216395 (PARI) a(n)=sum(k=2^(n-1), 2^n, vecsort(digits(k)) == vecsort(digits(sigma(k)))) \\ _V. Raman_, Feb 19 2014, based on edits by _M. F. Hasler_
%o A216395 (Python)
%o A216395 from sympy import divisor_sigma
%o A216395 def A216395(n):
%o A216395     if n == 1:
%o A216395         return 1
%o A216395     c = 0
%o A216395     for i in range(2**(n-1)+1, 2**n):
%o A216395         s1, s2 = sorted(str(i)), sorted(str(divisor_sigma(i)))
%o A216395         if len(s1) == len(s2) and s1 == s2:
%o A216395             c += 1
%o A216395     return c # _Chai Wah Wu_, Jul 23 2015
%Y A216395 Cf. A115920, A000203.
%K A216395 nonn,base,more
%O A216395 1,9
%A A216395 _V. Raman_, Sep 06 2012