Theorem 2.4. [1] Let Sn be the symmetric Pascal matrix of order n
defined by (1), then Sn is similar to its inverse S
−1
n
.
This result shows the following property of the eigenvalues of Sn.
Corollary 2.5. [1] Let Sn be the symmetric Pascal matrix of order
n defined by (1). Then the eigenvalues of Sn are pairs of reciprocal
numbers.
Proof. Let Sn be the symmetric Pascal matrix of order n defined by
(1) and λ be an eigenvalue of Sn. Since the det(Sn) = 1, we know Sn
is invertible. It follows that λ 6= 0 and, λ
−1
is an eigenvalue of S
−1
n
.
Since Sn and S
−1
n
are similar by Theorem 2.4, then Sn and S
−1
n have
the same eigenvalues. Hence, λ and λ
−1 are eigenvalues of Sn, and the
eigenvalues of Sn are pairs of reciprocal numbers.
Remark 1. If n is odd, since the eigenvalues must come in pairs, one
of the eigenvalues must be equal to 1.