cp's OEIS Frontend

A031954 Numbers with exactly two distinct base-9 digits.

%I A031954 #18 Jun 06 2023 03:52:51
%S A031954 9,11,12,13,14,15,16,17,18,19,21,22,23,24,25,26,27,28,29,31,32,33,34,
%T A031954 35,36,37,38,39,41,42,43,44,45,46,47,48,49,51,52,53,54,55,56,57,58,59,
%U A031954 61,62,63,64,65,66,67,68,69,71,72,73,74,75,76,77,78,79,81,82,90,92,93,94,95,96,97,98,100,101,109,111
%N A031954 Numbers with exactly two distinct base-9 digits.
%p A031954 isA031954 := proc(n)
%p A031954     local dgs,p,s ;
%p A031954     dgs := convert(convert(n,base,9),set) ;
%p A031954     if nops(dgs) = 2 then
%p A031954         true ;
%p A031954     else
%p A031954         false ;
%p A031954     end if ;
%p A031954 end proc:
%p A031954 A031954 := proc(n)
%p A031954     option remember ;
%p A031954     if n =1 then
%p A031954         9 ;
%p A031954     else
%p A031954         for a from procname(n-1)+1 do
%p A031954             if isA031954(a) then
%p A031954                 return a;
%p A031954             end if;
%p A031954         end do:
%p A031954     end if;
%p A031954 end proc:
%p A031954 seq(A031954(n),n=1..80) ; # _R. J. Mathar_, Jan 24 2023
%o A031954 (PARI) a(n)=n+8+(n+7)\9 \\ _Charles R Greathouse IV_, Mar 10 2021
%Y A031954 Cf. A007095.
%K A031954 nonn,base,easy
%O A031954 1,1
%A A031954 _Clark Kimberling_