This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A257631 #30 Oct 15 2021 06:06:04
%S A257631 10,15,21,91,171,1711
%N A257631 Near-repunit triangular numbers.
%C A257631 A near-repunit number is a number all but one of whose digits are 1's. No other near-repunit triangular number is known up to 10^15.
%C A257631 No more terms less than 10^1000. It is likely there are no more terms. - _Chai Wah Wu_, Mar 25 2020
%o A257631 (Python)
%o A257631 from sympy import integer_nthroot
%o A257631 def istri(n): return integer_nthroot(8*n+1, 2)[1]
%o A257631 def near_repunits(digits):
%o A257631 for loc in range(1, digits):
%o A257631 yield int("1"*loc + "0" + "1"*(digits-loc-1))
%o A257631 for loc in range(1, digits+1):
%o A257631 for d in "23456789":
%o A257631 yield int("1"*(digits-loc) + d + "1"*(loc-1))
%o A257631 def afind(maxdigits):
%o A257631 for digits in range(2, maxdigits+1):
%o A257631 for t in near_repunits(digits):
%o A257631 if istri(t): print(t, end=", ")
%o A257631 afind(200) # _Michael S. Branicky_, Oct 15 2021
%Y A257631 Cf. A000217, A002275, A062690, A257630.
%K A257631 base,nonn,more
%O A257631 1,1
%A A257631 _Shyam Sunder Gupta_, Jul 12 2015