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%I #7 Aug 11 2024 14:41:35
%S 2,4,6,7,9,11,16,18,19
%N Numbers k for which there exist no palindromic decagonal numbers (also known as 10-gonals) of length k.
%H P. De Geest, <a href="https://www.worldofnumbers.com/nona.htm">Palindromic decagonals</a>
%t A307827 = {0, 1, 232, 27972, 76867, 25555552, 7154664517, 158229922851, 2028787878202, 2040061600402, 2733623263372, 52667666676625, 675972505279576, 28519896169891582, 73542836563824537, 74529570707592547, 25552469511596425552, 27835145788754153872, 62740719088091704726, 67047523077032574076, 77979812588521897977, 107838025535520838701};
%t a[n_] := Length[Select[A307827, IntegerLength[#] == n || (n == 1 && # == 0) &]];
%t Select[Range[20], a[#] == 0 &]
%Y Cf. A001107, A082722, A307827, A307829.
%K nonn,base,hard
%O 1,1
%A _Robert Price_, Apr 30 2019