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.

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, 4208, 4156

Offset: 1

Author

V. Raman, Sep 06 2012

Keywords

• nonn
• base
• more

Examples

			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
		

Crossrefs

Cf. A115921, A000010.

Programs

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

Python

from sympy import totient
def A216394(n):
    if n == 1:
        return 1
    c = 0
    for i in range(2**(n-1)+1, 2**n):
        s1, s2 = sorted(str(i)), sorted(str(totient(i)))
        if len(s1) == len(s2) and s1 == s2:
            c += 1
    return c # Chai Wah Wu, Jul 23 2015

Formula

a(n) = # { k in A115921 | 2^(n-1) < k < 2^n }. - M. F. Hasler, Feb 24 2014