cp's OEIS Frontend

A379648 a(n) = position of prime(n) in A379652, or a(n) = -1 if prime(n) is not in A379652.

%I A379648 #6 Jan 01 2025 01:48:38
%S A379648 1,7,2,8,3,13,11,6,4,17,9,14,26,49,5,47,18,154,29,12,44,46,27,120,33,
%T A379648 16,102,48,43,36,10,87,21,52,41,15,71,86,28,58,121,31,114,172,45,190,
%U A379648 89,189,37,108,150,19,230,83,247,88,146,30,103,74,149,59,55,119
%N A379648 a(n) = position of prime(n) in A379652, or a(n) = -1 if prime(n) is not in A379652.
%H A379648 Michael De Vlieger, <a href="/A379648/b379648.txt">Table of n, a(n) for n = 1..10000</a>
%t A379648 c[_] := True; j = 2; c[2] = False; r = 0; a[1] = PrimePi[j];
%t A379648 Do[m = 2*j + 1;
%t A379648   While[
%t A379648     Set[k, SelectFirst[FactorInteger[m][[All, 1]], c]]; ! IntegerQ[k],
%t A379648     m = 2*m + 1];
%t A379648   c[k] = False; j = k; a[PrimePi[k]] = n;
%t A379648   If[k > r, r = k], {n, 2, 256}];
%t A379648 TakeWhile[Array[a, PrimePi[r]], IntegerQ]
%Y A379648 Cf. A379652.
%K A379648 nonn,easy
%O A379648 1,2
%A A379648 _Michael De Vlieger_, Jan 01 2025