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- [49] QUADRATIC INTEGRALS OF LINEAR HAMILTONIAN-SYSTEMS MONATSHEFTE FUR MATHEMATIK, 1984, 98 (01): : 53 - 63
- [50] NOTE ON THE NUMBER OF LIMIT-CYCLES IN CERTAIN TWO-DIMENSIONAL SYSTEMS JOURNAL OF THE LONDON MATHEMATICAL SOCIETY-SECOND SERIES, 1979, 20 (OCT): : 277 - 286