Verschueren, Robin and Zanon, Mario and Quirynen, Rien and Diehl, Moritz  A Sparsity Preserving Convexification Procedure for Indefinite Quadratic Programs Arising in Direct Optimal Control.  SIAM Journal on Optimization, 27 (3).  pp. 2085-2109.  ISSN 1052-6234      (2017)