The objective function is the standard deviation of the marginal risk
contributions, which is minimal, i.e. zero, if all
contributions are equal. The weights are rescaled to sum to unity.
References
Maillard, S. and Roncalli, T. and Teiletche, J.: The Properties
of Equally Weighted Risk Contribution Portfolios, Journal of
Portfolio Management, Vol. 36, No. 4, Summer 2010, 60--70.