OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Harvey P. Dale)
Eric Weisstein's World of Mathematics, Almost Prime.
Eric Weisstein's World of Mathematics, Heptagonal Number.
FORMULA
EXAMPLE
a(1) = 3 because Hep(3) = 3*(5*3-3)/2 = 18 = 2 * 3^2 is 3-almost prime.
a(2) = 8 because Hep(8) = 8*(5*8-3)/2 = 148 = 2^2 * 37 is 3-almost prime.
a(3) = 11 because Hep(11) = 11*(5*11-3)/2 = 286 = 2 * 11 * 13 is 3-almost prime.
a(17) = 82 because Hep(82) = 82*(5*82-3)/2 = 16687 = 11 * 37 * 41 is 3-almost prime (and 3-brilliant).
MATHEMATICA
Select[Range[400], PrimeOmega[# (5 # - 3)/2] == 3 &] (* Giovanni Resta, Jun 14 2016 *)
Select[Range[250], PrimeOmega[PolygonalNumber[7, #]]==3&] (* Harvey P. Dale, Sep 04 2020 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Feb 15 2006
EXTENSIONS
Corrected and extended by Giovanni Resta, Jun 14 2016
STATUS
approved