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 A226499 Triangular numbers representable as m * triangular(m). 3
 0, 1, 6, 4851 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS EXAMPLE 6 = 2 * triangular(2). 4851 = 21 * triangular(21). MATHEMATICA TriangularQ[n_] := IntegerQ[Sqrt[1 + 8*n]]; s = Select[Range[0, 10000], TriangularQ[#^2 (# + 1)/2] &]; s^2 (s + 1)/2 (* T. D. Noe, Jun 12 2013 *) PROG (Python) def isTriangular(a):     sr = 1 << (int.bit_length(int(a)) >> 1)     a += a     while a < sr*(sr+1):  sr>>=1     b = sr>>1     while b:       s = sr+b       if a >= s*(s+1):  sr = s       b>>=1     return (a==sr*(sr+1)) for n in range(10000):   product = n*n*(n+1)//2   if isTriangular(product):  print(product, end=', ') CROSSREFS Cf. A000217. Sequence in context: A209310 A268504 A099723 * A343096 A066061 A028366 Adjacent sequences:  A226496 A226497 A226498 * A226500 A226501 A226502 KEYWORD nonn AUTHOR Alex Ratushnyak, Jun 09 2013 STATUS approved

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Last modified May 28 00:30 EDT 2022. Contains 354110 sequences. (Running on oeis4.)