A378245 Numbers that are both k-gonal and (k+1)-gonal for some k >= 3.

1, 36, 1225, 9801, 40755, 41616, 121771, 297045, 631125, 1212751, 1413721, 2158695, 3617601, 5773825, 8851275, 13117251, 18886285, 26523981, 36450855, 48024900, 49146175, 65151801, 85076025, 94109401, 109597411, 139468635, 175520325, 218664901, 269900415, 330314391

Offset: 1

Author

Kelvin Voskuijl, Nov 20 2024

Keywords

• nonn

Examples

			a(2) = 36 is both the 8th triangular and the 6th square number.
a(3) = 1225 is both the 49th triangular and the 35th square number.
a(5) = 40755 is both the 165th pentagonal number and the 143th hexagonal number.
		

Crossrefs

Cf. A057145, A342300.

Cf. A001110, A036353, A046180, A048903, A048906, A048924 and A203627 (subsequences).

The subdiagonal of A189216 is also a subsequence.

Programs

PARI

upto(limit) = my(terms=List(1)); for(k=3, oo, my(found=0); for(n=2, oo, my(a = (2*n - 1)^2, b = (4*n*(3*n - 5) + 6), c = (8*(n-1)^2 + 1), s = (a*k^2 - b*k + c), v = n * (n*k - k - 2*n + 4) / 2); if(issquare(s), my(t = sqrtint(s) + k - 3); if(t % (2*(k-1)) == 0, listput(terms, v); found += 1)); if(v >= limit, break)); if(found == 0, break)); Vec(vecsort(terms)); \\ Daniel Suteu, Dec 08 2024

Extensions

a(12)-a(30) from Pontus von Brömssen, Dec 07 2024