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%I #14 May 13 2018 11:12:04
%S 1,21,3,45,55,6,78,820,91,10,1128,120,136,1431,15,1653,171,1830,190,
%T 2016,21,2211,231,2415,253,2628,276,28,2926,300,3160,325,3321,3403,
%U 351,36,378,3828,3916,406,4186,4278,435,4465,45,465,4753,4851,496,5050,5151,528
%N Smallest triangular number that begins with n.
%H Robert Israel, <a href="/A018855/b018855.txt">Table of n, a(n) for n = 1..10000</a>
%p f:= proc(n) local d,t;
%p for d from 0 do
%p t:= ceil((sqrt(8*n*10^d+1)+1)/2);
%p if (2*t-1)^2 < 8*(n+1)*10^d+1 then
%p return t*(t-1)/2 fi
%p od
%p end proc:
%p map(f, [$1..100]); # _Robert Israel_, Feb 27 2017
%t 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} ]
%t 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 *)
%Y Cf. A018801.
%K base,easy,nonn,look
%O 1,2
%A _David W. Wilson_
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