A192220 Semiprimes s such that tau(s-1) = tau(s+1) where tau = A000005.

4, 6, 9, 34, 51, 55, 86, 94, 129, 142, 183, 185, 202, 214, 218, 247, 249, 302, 341, 394, 415, 446, 471, 473, 535, 583, 634, 698, 723, 737, 807, 851, 905, 922, 926, 949, 1042, 1138, 1149, 1205, 1211, 1241, 1257, 1262, 1313, 1315, 1337, 1346, 1402, 1527, 1546, 1577, 1594, 1642, 1646, 1673, 1687

Offset: 1

Author

Juri-Stepan Gerasimov, Jun 26 2011

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000005, A001358, A067888, A067889.

Programs

Mathematica

Select[Range[2000], PrimeOmega[#] == 2 && Equal @@ DivisorSigma[0, # + {-1, 1}] &] (* Amiram Eldar, Jan 23 2025 *)

PARI

isok(k) = bigomega(k) == 2 && numdiv(k-1) == numdiv(k+1); \\ Amiram Eldar, Jan 23 2025

Formula

A001358 INTERSECT A067888.