A216391 Number of values of k for which phi(k) is a permutation of decimal digits of k, for k < 2^n.
1, 1, 1, 1, 2, 3, 3, 3, 5, 5, 5, 8, 19, 21, 34, 55, 81, 130, 221, 407, 515, 850, 1787, 2287, 3968, 8176, 12332, 18560, 36832, 71375, 94015, 173754
Offset: 1
Examples
a(12) = 8 because the values of k satisfying the condition for k < 2^12 are {1, 21, 63, 291, 502, 2518, 2817, 2991}. - _V. Raman_, Feb 18 2014
Programs
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Maple
A216391 := proc(n) local a,k,kdgs,pdgs ; a := 0 ; for k from 1 to 2^n do kdgs := sort(convert(k,base,10)) ; numtheory[phi](k) ; pdgs := sort(convert(%,base,10)) ; if pdgs = kdgs then a := a+1 ; end if; end do: a ; end proc: for n from 1 do print(A216391(n)) ; end do: # R. J. Mathar, Mar 04 2014
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PARI
a(n)=sum(k=1, 2^n, vecsort(digits(k)) == vecsort(digits(eulerphi(k)))) \\ V. Raman, Feb 18 2014, edited by M. F. Hasler, Mar 04 2014
Formula
a(n) = # { k in A115921 | k < 2^n }. - M. F. Hasler, Feb 24 2014
Extensions
a(28)-a(32) from Amiram Eldar, Nov 09 2024
Comments