|
Search: id:A072521
|
|
|
| A072521 |
|
a(1) = 6 and then the smallest triangular numbers such that sum of two neighbors is also a triangular number. |
|
+0
2
|
|
| 6, 15, 21, 45, 91, 990, 1711, 365085, 401856, 713415, 785631, 1079715, 1326006, 2355535, 2888406, 5137615, 5666661, 5764710, 9550635, 9921285, 10934826, 19434495, 21421785, 23622501, 42003195, 46315500, 82349361, 146384605
(list; graph; listen)
|
|
|
OFFSET
|
1,1
|
|
|
COMMENT
|
The sequence is unbounded as a(n+1) is less than or equal to the n-th triangular number.
|
|
EXAMPLE
|
45 is a term a 21 + 45 = 66 as well as 45 + 91 = 136 are triangular numbers.
|
|
PROGRAM
|
(PARI) p=6:k=3:for(n=1, 30, k=k+1:u=p+k*(k+1)/2:t=floor(sqrt(2*u)):while(u!=t*(t+1)/2, k=k+1:u=p+k*(k+1)/2:t=floor(sqrt(2*u))):p=k*(k+1)/2:print1(p", "))
|
|
CROSSREFS
|
Cf. A072522.
Sequence in context: A015793 A063466 A138109 this_sequence A130178 A100410 A095032
Adjacent sequences: A072518 A072519 A072520 this_sequence A072522 A072523 A072524
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Jul 31 2002
|
|
EXTENSIONS
|
More terms from Ralf Stephan (ralf(AT)ark.in-berlin.de), Mar 30 2003
|
|
|
Search completed in 0.002 seconds
|
