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%I #16 Aug 11 2024 14:41:34
%S 2,6,13,14,15,16,20,25,27,28,29,30,31,32
%N Numbers k for which there exist no palindromic 9-gonals (also known as nonagonals or enneagonals) of length k.
%C Previous name was: There exist no palindromic nonagonals (enneagonals) of length n.
%H P. De Geest, <a href="https://www.worldofnumbers.com/nona.htm">Palindromic nonagonals</a>
%t A082723 = {0, 1, 9, 111, 474, 969, 6666, 18981, 67276, 4411144, 6964696, 15444451, 57966975, 448707844, 460595064, 579696975, 931929139, 994040499, 1227667221, 9698998969, 61556965516, 664248842466, 699030030996, 99451743334715499, 428987160061789824, 950178723327871059, 1757445628265447571, 4404972454542794044, 9433971680861793349, 499583536595635385994, 1637992008558002997361, 19874891310701319847891};
%t A082722[n_] := Length[Select[A082723, IntegerLength[#] == n || (n == 1 && # == 0) &]];
%t Select[Range[22], A082722[#] == 0 &] (* _Robert Price_, Apr 29 2019 *)
%Y Cf. A034822, A059868, A082721, A059869, A059870, A034307.
%K nonn,base,hard
%O 1,1
%A _Patrick De Geest_, Apr 13 2003
%E Definition edited by _Jon E. Schoenfield_, Sep 15 2013