cp's OEIS Frontend

A217122 a(n) = (number of 1's in binary expansion of n)th positive number not among the previous terms.

%I A217122 #15 Jul 22 2025 23:38:47
%S A217122 1,2,4,3,6,7,9,5,10,11,13,12,15,16,18,8,17,19,21,20,23,24,26,22,27,28,
%T A217122 30,29,32,33,35,14,31,34,37,36,39,40,42,38,43,44,46,45,48,49,51,41,50,
%U A217122 52,54,53,56,57,59,55,60,61,63,62,65,66,68,25,58,64,69,67,71,72,74,70,75,76,78,77,80,81,83,73,82,84,86,85,88,89,91,87,92
%N A217122 a(n) = (number of 1's in binary expansion of n)th positive number not among the previous terms.
%C A217122 This is a permutation of the positive numbers.
%H A217122 Reinhard Zumkeller, <a href="/A217122/b217122.txt">Table of n, a(n) for n = 1..10000</a>
%H A217122 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a>
%e A217122 3 has 2 1's in its binary expansion, and the 2nd positive number not among the previous terms is 4; hence a(3)=4
%o A217122 (Haskell)
%o A217122 import Data.List (delete)
%o A217122 a217122 n = a217122_list !! (n-1)
%o A217122 a217122_list = f 1 [0..] where
%o A217122    f x zs = y : f (x + 1) (delete y zs) where
%o A217122      y = zs !! a000120 x
%o A217122 -- _Reinhard Zumkeller_, May 11 2013
%Y A217122 Cf. A000120: number of 1's in binary expansion of n.
%Y A217122 Cf. A225589 (inverse), A075311.
%K A217122 nonn,base
%O A217122 1,2
%A A217122 _Paul Tek_, Mar 16 2013