%I A028991
%S A028991 1,9,75,111,261,325,559,651,969,1089,1491,1639,2125,2301,2871,3075,
%T A028991 3729,3961,4699,4959,5781,6069,6975,7291,8281,8625,9699,10071,11229,
%U A028991 11629,12871,13299,14625,15081,16491,16975,18469,18981,20559,21099
%N A028991 Odd 9-gonal (or enneagonal) numbers.
%H A028991 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               NonagonalNumber.html">Source</a>
%F A028991 (1/2)[28n^2 + 4n + 1 + (14n+1)(-1)^n].
%Y A028991 Cf. A001106, A028992.
%Y A028991 Sequence in context: A145524 A037533 A001716 this_sequence A102094 A125397 
               A095249
%Y A028991 Adjacent sequences: A028988 A028989 A028990 this_sequence A028992 A028993 
               A028994
%K A028991 nonn
%O A028991 0,2
%A A028991 Patrick De Geest (pdg(AT)worldofnumbers.com)