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 A068812 Numbers n such that n and its 10's complement are both triangular numbers; that is, n and 10^k - n (where k is the number of digits in n) are triangular numbers. 1
 45, 55, 990, 1485, 4005, 4950, 5050, 5995, 8515, 9180, 285390, 499500, 500500, 714610, 1719585, 3064050, 6935950, 8280415, 49000050, 49995000, 50005000, 50999950, 1449668935, 4999950000, 5000050000, 8550331065, 122307408405, 122963116095 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Includes 5*10^(2*k+1)-5*10^k and 5*10^(2*k+1)+5*10^k for all k. - Robert Israel, Aug 14 2018 LINKS Robert Israel, Table of n, a(n) for n = 1..1890 EXAMPLE 1485 and 10000 - 1485 = 8515 both are triangular numbers. MAPLE f:= d -> op(sort(convert(select(t -> (t >= 10^(d-1) and t < 10^d),  map(t -> (t^2-1)/8, select(t -> t > 0, map(t -> subs(t, x), {isolve(x^2+y^2=8*10^d+2)})))), list))): seq(f(d), d=1..20); # Robert Israel, Aug 14 2018 MATHEMATICA Select[Accumulate[Range[50000]], IntegerQ[Sqrt[8*(10^(IntegerLength[#]) - #) + 1]] &] (* Jayanta Basu, Aug 05 2013 *) CROSSREFS Sequence in context: A168108 A037042 A119208 * A119071 A119158 A119204 Adjacent sequences:  A068809 A068810 A068811 * A068813 A068814 A068815 KEYWORD nonn,base,look AUTHOR Amarnath Murthy, Mar 07 2002 EXTENSIONS More terms from Sascha Kurz, Mar 17 2002 Seven additional terms from Jayanta Basu, Aug 05 2013 STATUS approved

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Last modified August 5 18:00 EDT 2021. Contains 346488 sequences. (Running on oeis4.)