A216395 - OEIS

%I #24 Jul 23 2015 22:21:28

%S 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 1726,3032

%N 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 a(n) = # { k in A115920 | 2^(n-1) < k < 2^n }. - _M. F. Hasler_, Feb 24 2014

%e 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 (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 (Python)

%o from sympy import divisor_sigma

%o def A216395(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(divisor_sigma(i)))

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

%o ............c += 1

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

%Y Cf. A115920, A000203.

%K nonn,base,more

%O 1,9

%A _V. Raman_, Sep 06 2012