1. An information processing apparatus, comprising: a memory; and a processor configured to use the memory and execute a process, the process comprising: generating a fitting curve represented by a sum of exponential functions for each of a set of digital inputs and a set of digital outputs for a physical system that is represented by one or a plurality of equations including m-order differential operators, wherein m is an integer equal to or greater than 1; and calculating coefficients of the differential operators, which are included in first coefficients, so that a first coefficient of each exponential function included in an expression obtained by a product of the differential operators and the fitting curve for the set of the digital inputs is equal to a second coefficient of the same exponential function, which is included in the fitting curve for the set of the digital outputs.