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(x0+x1i+x2j+x3k)=x0-x1i+x2j+x3k
