The result was redirect to Eigendecomposition of a matrix. King of ♥ ♦ ♣ ♠ 00:41, 22 March 2011 (UTC)[reply]
I work in linear algebra and I have never seen this basic fact called a "theorem" and given a proper name. A Google search seems to report no results as well, apart from Wikipedia and all the sites copying its content. Fph 21:36, 8 March 2011 (UTC)[reply]