Square Triangular Number

A number which is simultaneously square and triangular. Let  denote the th triangular number and  the th square number, then a number which is both triangular and square satisfies the equation , or

(1)

Completing the square gives

			(2)
			(3)
			(4)
			(5)

Therefore, defining

			(6)
			(7)

gives the Pell equation

(8)

(Conway and Guy 1996). The first few solutions are , (17, 12), (99, 70), (577, 408), .... These give the solutions , (8, 6), (49, 35), (288, 204), ... (OEIS A001108 and A001109), corresponding to the triangular square numbers 1, 36, 1225, 41616, 1413721, 48024900, ... (OEIS A001110; Pietenpol 1962). In 1730, Euler showed that there are an infinite number of such solutions (Dickson 2005).

The general formula for a square triangular number  is , where  is the th convergent to the continued fraction of  (Ball and Coxeter 1987, p. 59; Conway and Guy 1996). The first few are

(9)

(OEIS A001333 and A000129). The numerators and denominators can also be obtained by doubling the previous fraction and adding to the fraction before that.

A general formula for square triangular numbers is

			(10)
			(11)

The square triangular numbers also satisfy the recurrence relation

(12)

A second-order recurrence for  is given by

(13)

with  and . A first-order recurrence equation is given by

(14)

(M. Carreira, pers. comm., Sept. 29, 2003).

A curious product formula for  is given by

(15)

An amazing generating function is

(16)

(Sloane and Plouffe 1995).

Taking the square and triangular numbers together gives the sequence 1, 1, 3, 4, 6, 9, 10, 15, 16, 21, 25, ... (OEIS A005214; Hofstadter 1996, p. 15).

REFERENCES:

Allen, B. M. "Squares as Triangular Numbers." Scripta Math. 20, 213-214, 1954.

Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, 1987.

Conway, J. H. and Guy, R. K. The Book of Numbers. New York: Springer-Verlag, pp. 203-205, 1996.

Dickson, L. E. History of the Theory of Numbers, Vol. 2: Diophantine Analysis. New York: Dover, pp. 10, 16, and 27, 2005.

Guy, R. K. "Sums of Squares" and "Figurate Numbers." §C20 and §D3 in Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, pp. 136-138 and 147-150, 1994.

Hofstadter, D. R. Fluid Concepts & Creative Analogies: Computer Models of the Fundamental Mechanisms of Thought. New York: Basic Books, 1996.

Khatri, M. N. "Triangular Numbers Which are Also Squares." Math. Student 27, 55-56, 1959.

Pietenpol, J. L. "Square Triangular Numbers." Problem E 1473. Amer. Math. Monthly 69, 168-169, 1962.

Potter, D. C. D. "Triangular Square Numbers." Math. Gaz. 56, 109-110, 1972.

Sengupta, D. "Digits in Triangular Squares." College Math. J. 30, 31, 1999.

Sierpiński, W. Teoria Liczb, 3rd ed. Warsaw, Poland: Monografie Matematyczne t. 19, p. 517, 1950.

Sierpiński, W. "Sur les nombres triangulaires carrés." Pub. Faculté d'Électrotechnique l'Université Belgrade, No. 65, 1-4, 1961.

Sierpiński, W. "Sur les nombres triangulaires carrés." Bull. Soc. Royale Sciences Liège, 30 ann., 189-194, 1961.

Silverman, J. H. A Friendly Introduction to Number Theory. Englewood Cliffs, NJ: Prentice Hall, 1996.

Sloane, N. J. A. Sequences A000129/M1413, A001333/M2665, A001108/M4536, A001109/M4217, and A001110/M5259 in "The On-Line Encyclopedia of Integer Sequences."

Sloane, N. J. A. and Plouffe, S. The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.

Walker, G. W. "Triangular Squares." Problem E 954. Amer. Math. Monthly 58, 568, 1951.