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A249407 Numbers not in A249406. 4

%I #8 May 14 2017 11:56:06

%S 2,3,4,5,7,8,9,10,11,12,13,14,15,16,17,18,19,21,22,23,24,25,26,27,28,

%T 29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,

%U 52,53,54,55,57,58,59,60,61,62,63,64,65,66,67,68,69,70

%N Numbers not in A249406.

%C Primes are a subsequence, but semiprimes (cf. A001358) are not: first missing: A249406(1)=6=2*3, A249406(732)=2196323=1481*1483, A249406(1263)=6502499=2549*2551, A249406(1472)=8820899=2969*2971, A249406(1756)=12531599=3539*3541, ... ;

%C see A249411 for numbers of the form m*(m+1).

%o (Haskell)

%o import Data.List ((\\))

%o a249407 n = a249407_list !! (n-1)

%o a249407_list = f [2..] where

%o f ws@(u:v:_) = u : v : f (ws \\ [u, v, u * v])

%Y Cf. A249406 (complement), A000040 (subsequence), A001358.

%K nonn

%O 1,1

%A _Reinhard Zumkeller_, Oct 31 2014

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Last modified March 3 17:50 EST 2024. Contains 370512 sequences. (Running on oeis4.)