Aleksandrs Belovs Juris Smotrovs A Criterion for Attaining the Welch Bounds with Applications for Mutually Unbiased Bases. 50-69 2008 MMICS http://dx.doi.org/10.1007/978-3-540-89994-5_6 conf/mmics/2008 db/conf/mmics/mmics2008.html#BelovsS08