When is the 2 x 2 matrix ring over a commutative local ring strongly clean?

A ring R with identity is called strongly clean if every element of R is the sum of an idempotent and a unit that commute. Local rings are strongly clean. It is unknown when a matrix ring is strongly clean. However it is known from [J. Chen, X. Yang, Y. Zhou, On strongly clean matrix and triangular matrix rings, preprint, 2005] that for any prime number p, the 2 x 2 matrix ring M-2((Z) over capp) is strongly clean where (Z) over capp is the ring of p-adic integers, but M-2(Z((p))) is not strongly clean where Z((p)) is the localization of Z at the prime ideal generated by p. Let R be a commutative local ring. A criterion in terms of solvability of a simple quadratic equation in R is obtained for M2(R) to be strongly clean. As consequences, M-2(R) is strongly clean iff M-2(R[x]) is strongly clean iff M-2(R-[x]/(x(n))) is strongly clean iff M-2(RC2) is strongly clean. (c) 2005 Elsevier Inc. All rights reserved.