login
This site is supported by donations to The OEIS Foundation.

 

Logo

Invitation: celebrating 50 years of OEIS, 250000 sequences, and Sloane's 75th, there will be a conference at DIMACS, Rutgers, Oct 9-10 2014.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A085780 Numbers that are a product of 2 triangular numbers. 5
0, 1, 3, 6, 9, 10, 15, 18, 21, 28, 30, 36, 45, 55, 60, 63, 66, 78, 84, 90, 91, 100, 105, 108, 120, 126, 135, 136, 150, 153, 165, 168, 171, 190, 198, 210, 216, 225, 231, 234, 253, 270, 273, 276, 280, 300, 315, 325, 330, 351, 360, 378, 396, 406, 408, 420, 435, 441 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Is there a fast algorithm for detecting these numbers? - Charles R Greathouse IV, Jan 26 2013

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

EXAMPLE

18=t(2)*t(3)=3*6

MATHEMATICA

t1 = Table[n (n+1)/2, {n, 0, 100}]; Select[Union[Flatten[Outer[Times, t1, t1]]], # <= t1[[-1]] &] (* T. D. Noe, Jun 04 2012 *)

PROG

(PARI) A003056(n)=(sqrtint(8*n+1)-1)\2

list(lim)=my(v=List([0]), t); for(a=1, A003056(lim\1), t=a*(a+1)/2; for(b=a, A003056(lim\t), listput(v, t*b*(b+1)/2))); vecsort(Vec(v), , 8) \\ Charles R Greathouse IV, Jan 26 2013

CROSSREFS

Cf. A000217, A085782.

Sequence in context: A187577 A111359 A085782 * A166047 A223999 A107084

Adjacent sequences:  A085777 A085778 A085779 * A085781 A085782 A085783

KEYWORD

nonn

AUTHOR

Jon Perry, Jul 23 2003

EXTENSIONS

More terms from Max Alekseyev and Jon E. Schoenfield, Sep 04 2009

STATUS

approved

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

Content is available under The OEIS End-User License Agreement .

Last modified September 20 06:01 EDT 2014. Contains 246988 sequences.