OFFSET
0,2
COMMENTS
Never prime because a(n) = (2n^4 + 4n^3 + 6n^2 + 4n + 1) * (n^16 + 8n^15 + 76n^14 + 392n^13 + 1394n^12 + 3632n^11 + 7112n^10 + 10656n^9 + 12376n^8 + 11220n^7 + 7942n^6 + 4356n^5 + 1819n^4 + 560n^3 + 120n^2 + 16n + 1). Semiprime for n in {1, 13, 14, 54, 162, ...}. - Jonathan Vos Post, Aug 27 2011
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..10000
B. K. Teo and N. J. A. Sloane, Magic numbers in polygonal and polyhedral clusters, Inorgan. Chem. 24 (1985), 4545-4558.
EXAMPLE
a(1) = 1^20 + (1+1)^20 = 1048577 = 17 * 61681, which is semiprime.
PROG
(Magma) [(n+1)^20+n^20: n in [0..20]]; // Vincenzo Librandi, Aug 28 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved