login
A348423
Odd composite numbers k such that 2*k-3 is prime.
2
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
FORMULA
a(n) = (A348421(n) + 3)/2.
EXAMPLE
25 is a 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
Sequence in context: A036320 A339520 A340096 * A276290 A173251 A063149
KEYWORD
nonn,easy,changed
AUTHOR
Jianing Song, Oct 18 2021
STATUS
approved