OFFSET
1,1
COMMENTS
These are the values within the irreducible square roots, which are part of the area resulting from Heronian triangles, which have all sides being sequential odd integers (starting with the second odd number, since the triangle {1,3,5} has no real area). The triangles follow this sequence of sides: {2*n+1, 2*n+3, 2*n+5} and their area is represented by {(r/s)*sqrt(t)}, where r, s, t are integers and a(n) is the number t.
LINKS
Claudio Chaib, Squared area (squarefree): Triangles (OEIS-A334176 odd/A334177 prime), Wolfram Community.
Wikipedia, Heron's formula
FORMULA
a(n) = A007913(3*(2*n+7)*(2*n-1)). - Bernard Schott, Apr 18 2020
EXAMPLE
a(1) = 3 because the first possible Heronian triangle with its sequential odd integer sides is the triangle {3,5,7} and its respective area is {15*sqrt(3)/4}. a(2) = 11 since the second possible triangle is {5,7,9} which has area {21*sqrt(11)/4}. And so on.
MATHEMATICA
a[n_]:=Module[{y, z}, z=Area@SSSTriangle[2*n+1, 2*n+3, 2*n+5]; ({z}/.Coefficient[{z}/.Sqrt[_]->y, y][[1]]->1)[[1]]^2]
PROG
(PARI) a(n) = core((3*(2*n+7)*(2*n-1))); \\ Michel Marcus, Apr 18 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Claudio Lobo Chaib Filho, Apr 17 2020
STATUS
approved