Explicit formulas
Write for the th square triangular number, and write and for the sides of the corresponding square and triangle, so that : Define the ''triangular root'' of a triangular number to be . From this definition and the quadratic formula, : Therefore, is triangular ( is an integer) if and only if is square. Consequently, a square number is also triangular if and only if is square, that is, there are numbers and such that . This is an instance of the Pell equation with . All Pell equations have the trivial solution for any ; this is called the zeroth solution, and indexed as . If denotes the th nontrivial solution to any Pell equation for a particular , it can be shown by the method of descent that : Hence there are an infinity of solutions to any Pell equation for which there is one non-trivial one, which holds whenever is not a square. The first non-trivial solution when is easy to find: it is (3,1). A solution to the Pell equation for yields a square triangular number and its square and triangular roots as follows: : Hence, the first square triangular number, derived from (3,1), is 1, and the next, derived from , is 36. The sequences , and are the OEIS sequences , , and respectively. In 1778 Leonhard Euler determined the explicit formula : Other equivalent formulas (obtained by expanding this formula) that may be convenient include : The corresponding explicit formulas for and are: :Pell's equation
The problem of finding square triangular numbers reduces to Pell's equation in the following way. Every triangular number is of the form . Therefore we seek integers , such that : Rearranging, this becomes : and then letting and , we get theRecurrence relations
There are recurrence relations for the square triangular numbers, as well as for the sides of the square and triangle involved. We have : We have :Other characterizations
All square triangular numbers have the form , where is a convergent to the continued fraction expansion of . A. V. Sylwester gave a short proof that there are an infinity of square triangular numbers: If the th triangular number is square, then so is the larger th triangular number, since: : As the product of three squares, the right hand side is square. The triangular roots are alternately simultaneously one less than a square and twice a square if is even, and simultaneously a square and one less than twice a square if is odd. Thus, :49 = 72 = 2 × 52 − 1, :288 = 172 − 1 = 2 × 122, and :1681 = 412 = 2 × 292 − 1. In each case, the two square roots involved multiply to give : , , and . Additionally: : , , and . In other words, the difference between two consecutive square triangular numbers is the square root of another square triangular number. TheNumerical data
As becomes larger, the ratio approaches ≈ , and the ratio of successive square triangular numbers approaches ≈ . The table below shows values of between 0 and 11, which comprehend all square triangular numbers up to . :See also
* Cannonball problem, on numbers that are simultaneously square and square pyramidal * Sixth power, numbers that are simultaneously square and cubicalNotes
External links