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- [11] A COMPARISON PRINCIPLE FOR 2ND ORDER DIFFERENTIAL EQUATIONS DOKLADY AKADEMII NAUK SSSR, 1960, 135 (04): : 783 - 786
- [12] DISCONJUGATE DIFFERENTIAL EQUATIONS OF 2ND ORDER IN COMPLEXES JOURNAL FUR DIE REINE UND ANGEWANDTE MATHEMATIK, 1972, 257 : 1 - &