A256420 - OEIS

A256420

a(n) = 2n unless n is prime, in which case a(n) = first term not yet present in the sequence.

%I #21 May 18 2020 19:27:03

%S 2,1,3,8,4,12,5,16,18,20,6,24,7,28,30,32,9,36,10,40,42,44,11,48,50,52,

%T 54,56,13,60,14,64,66,68,70,72,15,76,78,80,17,84,19,88,90,92,21,96,98,

%U 100,102,104,22,108,110,112,114,116,23,120

%N a(n) = 2n unless n is prime, in which case a(n) = first term not yet present in the sequence.

%C This is a permutation of the positive integers: twice nonprimes (A139270), interspersed with (odd numbers and twice primes, A256421).

%D John Mason, Email message, Apr 07 2015

%H Robert Israel, <a href="/A256420/b256420.txt">Table of n, a(n) for n = 1..10000</a>

%p N:= 100:

%p S:= {$1..N}:

%p R:= NULL;

%p for n from 1 do

%p if isprime(n) then if S = {} then break else t:= min(S) fi else t:= 2*n fi;

%p R:= R, t;

%p S:= S minus {t}

%p od:

%p R;

%Y Cf. A139270, A256421.

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Apr 07 2015