| There are certainly mathematical logicians who are formalists, even in the light of the incompleteness theorems. |
| He summed series, and discovered addition theorems for trigonometric and hyperbolic functions using the differential equations they satisfy. |
| Moreover, the inability to assert theorems containing free variables makes it impossible to prove any de re modal validities. |
| He worked on conic sections and produced important theorems in projective geometry. |
| It contained a number of projective geometry theorems, including Pascal's mystic hexagon. |
| And quite frequently I state a number of definitions and ask students to formulate some theorems using them. |
