Laplacian using SDP

Is there any suggestion about how could one construct a model that uses semidefinite programming that minimizes sum of k smallest eigenvalues of Laplacian matrix? I found two papers that have done something for the just bound not for optimization.

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minimizing sum of $k$-largest is a convex problem; the sum of $k$-smallest eigenvalues is concave, and therefore, not really that conducive to minimization... –  Suvrit Feb 4 '13 at 22:35
Thanks a lot for the answer. i tried another way using the maximization of k-largest eigenvalues. –  Royeh Feb 23 '13 at 0:03