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A018855 Smallest triangular number that begins with n. 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

MAPLE

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:

map(f, [$1..100]); # Robert Israel, Feb 27 2017

MATHEMATICA

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 *)

CROSSREFS

Cf. A018801.

Sequence in context: A080472 A118389 A077690 * A263277 A040428 A040429

Adjacent sequences:  A018852 A018853 A018854 * A018856 A018857 A018858

KEYWORD

base,easy,nonn,look

AUTHOR

David W. Wilson

STATUS

approved

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Last modified September 20 05:51 EDT 2019. Contains 327212 sequences. (Running on oeis4.)