A176069 Numbers of the form k^2+k+1 that are the product of two distinct primes.

21, 57, 91, 111, 133, 183, 381, 553, 703, 813, 871, 993, 1057, 1191, 1261, 1333, 1561, 1641, 1807, 1893, 1981, 2071, 2257, 2353, 2653, 2757, 2863, 3193, 3661, 4033, 4291, 4971, 5257, 5403, 5853, 6807, 6973, 7141, 7311, 7483, 8373, 8557, 8743, 9121, 9313, 9507, 9703

Offset

1,1

Links

David A. Corneth, Table of n, a(n) for n = 1..10000

Example

21 is a term as 21 = 3*7 = 4^2+4+1; 21 is the product of two distinct primes and 21 is of the form k^2 + k + 1.

Mathematica

f[n_]:=Last/@FactorInteger[n]=={1, 1}; Select[Array[ #^2+#+1&, 6!, 2], f[ # ]&]

Crossrefs

Cf. A002061, A006881, A174969.

Sequence in context: A257103 A146560 A174969 * A043382 A044123 A044504

Adjacent sequences: A176066 A176067 A176068 * A176070 A176071 A176072

Keyword

nonn

Author

Vladimir Joseph Stephan Orlovsky, Apr 07 2010

Extensions

Name corrected by David A. Corneth, May 29 2023

Status

approved