Zelikin, M. I.; Borisov, V. F. (2005). "Singular Optimal Regimes in Problems of Mathematical Economics". Journal of Mathematical Sciences. 130 (1): 4409–4570 [Theorem 11.1]. doi:10.1007/s10958-005-0350-5. S2CID 122382003. /wiki/Mikhail_Zelikin ↩