OFFSET
1,2
COMMENTS
The sequence is closed under multiplication. Primitive elements are 3 and the primes of form 3*k+2.
a(n)^2 is not expressible as x^2+xy+y^2 with x and y positive integers.
Analog of A004144 (nonhypotenuse numbers) for 120-degree angle triangles: a(n) is not the length of the longest side of such a triangle with integer sides.
It might have been natural to include 0 in this sequence. - M. F. Hasler, Mar 04 2018
LINKS
Ray Chandler, Table of n, a(n) for n = 1..10000 (first 254 terms from Jean-Christophe Hervé)
F. Javier de Vega, An extension of Furstenberg's theorem of the infinitude of primes, arXiv:2003.13378 [math.NT], 2020.
August Lösch, Economics of Location (1954), see pp. 117f.
U. P. Nair, Elementary results on the binary quadratic form a^2+ab+b^2, arXiv:math/0408107 [math.NT], 2004.
FORMULA
A005088(a(n)) = 0.
MATHEMATICA
Join[{1}, Select[Range[2, 110], ! MemberQ[Union[Mod[Transpose[ FactorInteger[#]][[1]], 6]], 1] &]] (* T. D. Noe, Nov 24 2013 *)
Join[{1}, Select[Range[110], NoneTrue[FactorInteger[#][[All, 1]], Mod[#, 6] == 1&]&]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Feb 03 2019 *)
PROG
(Haskell)
a230780 n = a230780_list !! (n-1)
a230780_list = filter (all (/= 1) . map (flip mod 6) . a027748_row) [1..]
-- Reinhard Zumkeller, Apr 09 2014
(PARI) is_A230780(n)=!setsearch(Set(factor(n)[, 1]%6), 1) \\ M. F. Hasler, Mar 04 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Jean-Christophe Hervé, Nov 23 2013
STATUS
approved