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%I #20 Dec 02 2023 07:48:48
%S 1,3,6,21,36,66,91,136,406,666,703,903,3003,6903,7021,8001,5000703
%N Left-truncatable nonzero triangular numbers.
%C Leading zeros in postfix strings are ignored (e.g., 003 and 03 are both equivalent to 3). Zero was excluded from the set because otherwise any integer starting with 1, 3, or 6 followed by any number of zeros would be a member.
%C Compare this sequence with the right-truncatable triangular numbers listed in A202269.
%C Conjecture: this sequence appears to be finite and full (brute-force tested up to 228*10^9 digits).
%C Both the source number and the result must be nonzero, otherwise 10 would be a term of the sequence. - _Georg Fischer_, Dec 02 2023
%e 6903 is a term because it is a nonzero triangular number, and 903 is a term of the sequence.
%p isA300891 := proc(n)
%p option remember ;
%p if n in {1,3,6} then
%p return true;
%p elif n < 10 then
%p return false;
%p end if;
%p if isA000217(n) then
%p dgs := max(1,ilog10(n)+1) ;
%p return procname( modp(n,10^(dgs-1))) ;
%p else
%p return false;
%p end if;
%p end proc:
%p for i from 1 do
%p t := A000217(i) ;
%p if isA300891(t) then
%p print(t) ;
%p end if;
%p end do: # _R. J. Mathar_, May 02 2018
%Y Cf. A000217, A202269.
%K nonn,base,more
%O 1,2
%A _Stanislav Sykora_, Mar 14 2018
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