I've been reading two (peer reviewed) papers that use Principal Component Analysis to solve a problem I'm interested in, they both state they find the eigenvalues for the correlation matrix using the equation
$$ | \lambda I - A| = 0. $$
When I've read around PCA and eigenvalue decomposition, the appropriate equation seems to be defined as
$$ | A - \lambda I | = 0. $$
Could anyone explain to me why there might be a difference, and what the impact of this may be?