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%I #15 Aug 14 2018 21:03:12
%S 45,55,990,1485,4005,4950,5050,5995,8515,9180,285390,499500,500500,
%T 714610,1719585,3064050,6935950,8280415,49000050,49995000,50005000,
%U 50999950,1449668935,4999950000,5000050000,8550331065,122307408405,122963116095
%N 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.
%C 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
%H Robert Israel, <a href="/A068812/b068812.txt">Table of n, a(n) for n = 1..1890</a>
%e 1485 and 10000 - 1485 = 8515 both are triangular numbers.
%p 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),
%p {isolve(x^2+y^2=8*10^d+2)})))),list))):
%p seq(f(d),d=1..20); # _Robert Israel_, Aug 14 2018
%t Select[Accumulate[Range[50000]], IntegerQ[Sqrt[8*(10^(IntegerLength[#]) - #) + 1]] &] (* _Jayanta Basu_, Aug 05 2013 *)
%K nonn,base,look
%O 1,1
%A _Amarnath Murthy_, Mar 07 2002
%E More terms from _Sascha Kurz_, Mar 17 2002
%E Seven additional terms from _Jayanta Basu_, Aug 05 2013
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