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%I #26 Oct 15 2024 14:36:50
%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;
%o (Python)
%o from sympy import primepi, isprime
%o def A256420(n):
%o r = int(primepi(n))
%o def iterfun(f,n=0):
%o m, k = n, f(n)
%o while m != k: m, k = k, f(k)
%o return m
%o def f(x): return int(r+(m:=x>>1)-primepi(m))
%o return iterfun(f,r) if isprime(n) else n<<1 # _Chai Wah Wu_, Oct 15 2024
%Y Cf. A139270, A256421.
%K nonn
%O 1,1
%A _N. J. A. Sloane_, Apr 07 2015