635459076096domain: NAppears in sequencesReciprocal Chebyshev polynomial of second kind evaluated at 4 multiplied by (-1)^n.at n=20A025171a(n) = 4^n sin(nC)/sqrt(15), where B is the angle opposite side CA in a triangle ABC having sidelengths |BC|=2, |CA| = 3, |AB| = 4; ABC is the smallest integer-sided scalene triangle.at n=21A375885