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
Keywords
Examples
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.
Links
- David A. Corneth, Table of n, a(n) for n = 1..10000
Programs
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Mathematica
f[n_]:=Last/@FactorInteger[n]=={1,1};Select[Array[ #^2+#+1&,6!,2],f[ # ]&]
Extensions
Name corrected by David A. Corneth, May 29 2023