Relationship between Chebyshev U and Gegenbauer C

Theorem

The following formula holds for $n \in \{1,2,3,\ldots\}$: $$U_n(x)=\sqrt{1-x^2}C_{n-1}^1(x),$$ where $U_n$ denotes a Chebyshev polynomial of the second kind and $C_{n-1}^1$ denotes a Gegenbauer C polynomial.