A336429 First location of n in A331025 or 0 if number is absent.

1, 0, 2, 3, 5, 7, 11, 13, 4, 17, 19, 6, 23, 29, 10, 9, 31, 14, 37, 15, 22, 41, 43, 21, 25, 47, 8, 33, 53, 34, 59, 39, 38, 61, 55, 12, 67, 71, 46, 51, 73, 58, 79, 57, 20, 83, 89, 18, 121, 85, 62, 69, 97, 28, 95, 87, 74, 101, 103, 30, 107, 109, 44, 27, 115, 82

Offset: 1

Author

J. Lowell, Jul 21 2020

Keywords

• nonn

Comments

26 is the smallest number not in this sequence; A331025(21) and A331025(26) both equal 24; the smallest number to occur in A331025 at least twice.

Examples

			a(9) = 4 because A331025(4) = 9 and no earlier term of A331025 is 9.
		

Crossrefs

Cf. A232803, A331025.

Programs

Mathematica

Block[{nn = 66, s}, s = Select[Range[3 nn], And[# != 2, Or[Log2[#] == 3, PrimeQ@ #, PrimeQ[#/2]]] &]; Insert[#, 0, 2][[1 ;; nn]] &@ Values[KeySort@ PositionIndex@ Array[Times @@ Map[If[#[[1]] == 1, 1, # /. {p_, e_} :> s[[PrimePi@ p]]^e] &, FactorInteger[#]] &, Prime@ Length@ s]][[All, 1]]] (* Michael De Vlieger, Aug 21 2020 *)

PARI

isp(n) = (isprime(n) && (n%2)) || (n==8) || (!(n%2) && isprime(n/2)); \\ A232803
vA331025(nn) = {my(vall = [1..nn]); my(vp = select(x->isp(x), vall)); for (n=2, nn, my(f=factor(n)); for (k=1, #f~, f[k,1] = vp[primepi(f[k,1])]); vall[n] = factorback(f);); vall;}
lista(nn) = {my(vall = vA331025(nn)); my(vr = vector(nn)); for (n=1, nn, my(vs = select(x->(x==n), vall, 1)); if (#vs == 0, vr[n] = 0, vr[n] = vs[1]);); my(vz = select(x->(x==0), vr, 1)); if (#vz > 1, vr = vector(vz[2]-1, k, vr[k]);); vr;} \\ Michel Marcus, Sep 14 2020

Extensions

More terms from Rémy Sigrist, Jul 21 2020