OFFSET
1,1
REFERENCES
J. H. Conway and R. K. Guy, The Book of Numbers, New York, Springer-Verlag, p. 50, 1996.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
Hyun Kwang Kim, On Regular Polytope Numbers, Proc. Amer. Math. Soc., 131 (2003), 65-75.
Eric Weisstein's World of Mathematics, Semiprime.
FORMULA
EXAMPLE
MATHEMATICA
Select[ Prime[ Range[ 557]], PrimeQ[(5#^2 - 5# + 2)/2] &] (* Robert G. Wilson v, Feb 21 2005 *)
PROG
(PARI) isok(n) = bigomega(n*(5*n^2 -5*n + 2)/2) == 2; \\ Michel Marcus, Dec 14 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jonathan Vos Post, Feb 20 2005
EXTENSIONS
Edited and extended by Robert G. Wilson v, Feb 21 2005
STATUS
approved