D'Alembert's and Wilson's functional equations for vector and $2\times 2$ matrix valued functions

H. Stetkær, “D’Alembert’s and Wilson’s functional equations for vector and $2\times 2$ matrix valued functions”, Math. Scand., vol. 87, no. 1, pp. 115–132, Sep. 2000, doi: 10.7146/math.scand.a-14302.