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A018855
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Smallest triangular number that begins with n.
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4
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1, 21, 3, 45, 55, 6, 78, 820, 91, 10, 1128, 120, 136, 1431, 15, 1653, 171, 1830, 190, 2016, 21, 2211, 231, 2415, 253, 2628, 276, 28, 2926, 300, 3160, 325, 3321, 3403, 351, 36, 378, 3828, 3916, 406, 4186, 4278, 435, 4465, 45, 465, 4753, 4851, 496, 5050, 5151, 528
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OFFSET
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1,2
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LINKS
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MAPLE
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f:= proc(n) local d, t;
for d from 0 do
t:= ceil((sqrt(8*n*10^d+1)+1)/2);
if (2*t-1)^2 < 8*(n+1)*10^d+1 then
return t*(t-1)/2 fi
od
end proc:
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MATHEMATICA
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Do[k = Ceiling[ Sqrt[n]]; While[ FromDigits[ Take[ IntegerDigits[ k(k + 1)/2], Floor[ Log[10, n] + 1]]] != n, k++ ]; Print[ k(k + 1)/2], {n, 1, 50} ]
With[{tr=Accumulate[Range[150]]}, Table[SelectFirst[tr, Take[ IntegerDigits[ #], Length[ IntegerDigits[ n]]] == IntegerDigits[n]&], {n, 60}]]//Quiet (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 13 2018 *)
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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