A338309 a(1) = 4; thereafter a(n) is the least new semiprime such that a(n-1) + a(n) is a semiprime.

4, 6, 9, 25, 10, 15, 34, 21, 14, 35, 22, 33, 49, 38, 39, 26, 51, 55, 74, 69, 46, 65, 57, 58, 85, 93, 62, 115, 86, 91, 87, 82, 77, 106, 95, 111, 94, 119, 118, 129, 133, 121, 141, 146, 143, 122, 145, 142, 123, 155, 134, 161, 158, 169, 166, 205, 177, 178, 183, 194, 187, 159, 202, 201

Offset

1,1

Comments

It is conjectured that every semiprime will appear.

Links

Rémy Sigrist, Table of n, a(n) for n = 1..10000

Rémy Sigrist, PARI program for A338309

Example

4+6 = 10 = A001358(4), 6+9 = 15 = A001358(6), 9+25 = 34 = A001358(12).

Mathematica

sp = Select[Range[4, 500], 2 == PrimeOmega[#] &]; s = {4}; a = 4;
Do[Do[b = sp[[k2]]; If[FreeQ[s, b] && 2 == PrimeOmega[a + b], AppendTo[s, (a = b)]; Break[]], {k2, Length[sp]}], {k1, 70}]; s

Prog

(PARI) See Links section.

Crossrefs

Cf. A001358.

Sequence in context: A046376 A229129 A272436 * A246569 A368648 A326063

Adjacent sequences: A338306 A338307 A338308 * A338310 A338311 A338312

Keyword

nonn

Author

Zak Seidov, Oct 22 2020

Status

approved