login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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
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 * A349873 A263277 A040428
KEYWORD
base,easy,nonn,look
AUTHOR
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 04:13 EDT 2024. Contains 371235 sequences. (Running on oeis4.)