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A216396
Number of values of k for which sigma(k)-k is a permutation of decimal digits of k, for 2^(n-1) < k < 2^n.
1
0, 0, 1, 0, 1, 0, 0, 0, 2, 1, 1, 2, 5, 4, 15, 15, 16, 35, 91, 90, 158, 345, 586, 694, 1549, 2700, 3363
OFFSET
1,9
FORMULA
a(n) = # { k in A085844 | 2^(n-1) < k < 2^n }. - M. F. Hasler, Feb 24 2014
EXAMPLE
a(13) = 5 because the values of k satisfying the condition for 2^12 < k < 2^13 are {4672, 4896, 5046, 7785, 8128}. - V. Raman, Feb 19 2014
PROG
(PARI) a(n)=sum(k=2^(n-1), 2^n, vecsort(digits(k)) == vecsort(digits(sigma(k)-k))) \\ V. Raman, Feb 19 2014, based on edits by M. F. Hasler
(Python)
from sympy import divisor_sigma
def A216396(n):
....c = 0
....for i in range(2**(n-1)+1, 2**n):
........s1, s2 = sorted(str(i)), sorted(str(divisor_sigma(i)-i))
........if len(s1) == len(s2) and s1 == s2:
............c += 1
....return c # Chai Wah Wu, Jul 23 2015
CROSSREFS
Sequence in context: A289772 A283615 A172483 * A273488 A334955 A117848
KEYWORD
nonn,base,more
AUTHOR
V. Raman, Sep 06 2012
STATUS
approved