A348423 Odd composite numbers k such that 2*k-3 is prime.

25, 35, 55, 65, 77, 85, 91, 115, 121, 133, 143, 155, 175, 185, 217, 221, 235, 245, 247, 253, 275, 287, 295, 301, 305, 323, 325, 343, 361, 365, 371, 377, 395, 407, 413, 415, 445, 455, 475, 485, 493, 497, 511, 517, 527, 533, 545, 553, 583, 595, 655, 665, 685

Offset

1,1

Links

Jianing Song, Table of n, a(n) for n = 1..10000

Formula

a(n) = (A348421(n) + 3)/2.

Example

25 is term since 25 is an odd composite number and 2*25 - 3 = 47 is prime.

Mathematica

Select[Range[1, 700, 2], CompositeQ[#] && PrimeQ[2*# - 3] &] (* Amiram Eldar, Oct 18 2021 *)

Prog

(PARI) isA348423(n) = !isprime(n) && (n%2) && isprime(2*n-3)

Crossrefs

Cf. A002145, A348421.

Sequence in context: A036320 A339520 A340096 * A276290 A173251 A063149

Adjacent sequences: A348420 A348421 A348422 * A348424 A348425 A348426

Keyword

nonn,easy

Author

Jianing Song, Oct 18 2021

Status

approved