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Heptagonal Triangular Number


A number which is simultaneously a heptagonal number H_n and triangular number T_m. Such numbers exist when

 1/2n(5n-3)=1/2m(m+1).
(1)

Completing the square and rearranging gives

 (10n-3)^2-5(2m+1)^2=4.
(2)

Substituting x=10n-3 and y=2m+1 gives the Pell-like quadratic Diophantine equation

 x^2-5y^2=4,
(3)

which has basic solutions (x,y)=(3,1), (7, 3), and (18, 8). Additional solutions can be obtained from the unit Pell equation, and correspond to integer solutions when (n,m)=(1,1), (5, 10), (221, 493), (1513, 3382), ... (OEIS A046193 and A039835), corresponding to the heptagonal triangular numbers 1, 55, 121771, 5720653, 12625478965, ... (OEIS A046194).


See also

Heptagonal Number, Triangular Number

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References

Sloane, N. J. A. Sequences A039835, A046193, and A046194 in "The On-Line Encyclopedia of Integer Sequences."

Referenced on Wolfram|Alpha

Heptagonal Triangular Number

Cite this as:

Weisstein, Eric W. "Heptagonal Triangular Number." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HeptagonalTriangularNumber.html

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