A257630 Near-repdigit triangular numbers.
10, 15, 21, 28, 36, 45, 78, 91, 171, 300, 595, 990, 1711, 5565, 6555, 66066, 333336
Offset: 1
Programs
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Mathematica
nrepQ[n_] := Module[{dg = Select[DigitCount[n], # > 0 &]},Length[dg] == 2 && Min[dg] == 1 && Max[dg] > 0]; Select[ Table[n*(n + 1)/2, {n, 10000}], nrepQ] -
Python
from sympy import integer_nthroot def istri(n): return integer_nthroot(8*n+1, 2)[1] def near_repdigits(digits): s = set() for d1 in "0123456789": for d2 in set("0123456789") - {d1}: for loc in range(1, digits+1): nrd = d1*(digits-loc) + d2 + d1*(loc-1) if nrd[0] != "0": s.add(int(nrd)) return sorted(s) def afind(maxdigits): for digits in range(2, maxdigits+1): for t in near_repdigits(digits): if istri(t): print(t, end=", ") afind(100) # Michael S. Branicky, Oct 15 2021
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