login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A296098 a(n) is the smallest triangular number (A000217) that can be represented as a product of k triangular numbers greater than 1 for all k = 1,2,...,n. a(n)=-1 if no such triangular number exists. 0
3, 36, 630, 25200, 25200, 2821500, 55030954895280, 55030954895280, 55030954895280, 55030954895280, 55030954895280, 55030954895280 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

From Jon E. Schoenfield, Apr 21 2018: (Start)

Of the 482 triangular numbers < 55030954895280 that can be represented as a product of seven triangular numbers greater than 1, the only one that can be represented as a product of two triangular numbers greater than 1 is 218434391280, which cannot be represented as a product of 3 triangular numbers greater than 1. Thus, a(n) >= 55030954895280 for all n >= 7.

However, 55030954895280 can be represented (see Example section) as a product of k triangular numbers greater than 1 for all k in 1,2,...,12 (but not for k=13), so a(7) = a(8) = ... = a(12) = 55030954895280 (and, for each n > 12, a(n) > 55030954895280, or a(n) = -1 if no such number exists).

If, for some integer N > 12, it could be proved that a(N) = -1, then it would also be established that a(n) = -1 for every n > N. (End)

LINKS

Table of n, a(n) for n=1..12.

FORMULA

a(n) >= A212616(n) (unless a(n) = -1). - Jon E. Schoenfield, Apr 21 2018

EXAMPLE

25200 is the smallest triangular number representable as a product of 2, 3 and 4 triangular numbers, 25200 = 210 * 120 = 120 * 21 * 10 = 28 * 15 * 10 * 6.

Therefore a(4)=25200.

Also, 25200 = 28 * 10 * 10 * 3 * 3, and therefore a(5)=25200.

From Jon E. Schoenfield, Apr 21 2018: (Start)

Let f(k_1, k_2, ..., k_m) = Product_{j=1..m} A000217(k_j) = Product_{j=1..m} (k_j*(k_j + 1)/2). Then, since no smaller number can be represented as a product of k triangular numbers greater than 1 for all k in 1,2,...,7,

a(7) = 55030954895280

     = f(10491039)

     = f(2261, 6560)

     = f(6, 493, 6560)

     = f(28, 39, 81, 323)

     = f(17, 18, 27, 40, 116)

     = f(4, 8, 17, 28, 38, 81)

     = f(2, 3, 17, 18, 26, 28, 40)

     = f(2, 2, 2, 2, 2, 17, 144, 532)

     = f(2, 2, 2, 2, 12, 17, 18, 28, 40)

     = f(2, 2, 2, 2, 2, 2, 3, 3, 40, 2261)

     = f(2, 2, 2, 2, 2, 2, 2, 2, 16, 29, 532)

     = f(2, 2, 2, 2, 2, 2, 2, 2, 2, 7, 40, 493)

and a(7) = a(8) = a(9) = a(10) = a(11) = a(12).

(End)

CROSSREFS

Cf. A000217, A188630, A212616, A295769, A296097.

Sequence in context: A234869 A305991 A203493 * A224256 A223912 A224184

Adjacent sequences:  A296095 A296096 A296097 * A296099 A296100 A296101

KEYWORD

nonn,more

AUTHOR

Alex Ratushnyak, Dec 04 2017

EXTENSIONS

a(7)-a(12) from Jon E. Schoenfield, Apr 21 2018

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 6 15:41 EDT 2020. Contains 333276 sequences. (Running on oeis4.)