
REFERENCES

D. Ballew and R. Weger, Repdigit triangular numbers, J. Rec. Math. Vol. 8(2), p. 96, 197576.
L. E. Dickson, History of the Theory of Numbers, Vol. II, p. 33, Chelsea NY, 1952.
E. B. Escott, Math. Quest. Educational Times, (2) 8 (1905), 3334.  N. J. A. Sloane, Mar 31 2014
Bir Kafle et al., Triangular repblocks, Fib. Q., 56:4 (2018), 325328.


MATHEMATICA

Select[Union[Flatten[Table[FromDigits[PadRight[{}, n, k]], {n, 3}, {k, 0, 9}]]], OddQ[ Sqrt[8#+1]]&] (* Harvey P. Dale, Feb 11 2020 *)
