Because I explore calculus over division ring I put attention on different papers related to quaternion calculus. These papers mainly discuss definition of regular function. According to definition regular function is function which satisfy to differential equation^{∂f}/_{∂x0}+i^{∂f}/_{∂x1}+j^{∂f}/_{∂x2}+k^{∂f}/_{∂x3}=0

Recently Daniel Alayon-Solarz sent me interesting example of regular function.

f(x_{0}+x_{1}i+x_{2}j+x_{3}k)=x_{0}-x_{1}i+x_{2}j+x_{3}k