  An analytical relation between center of mass momenta in a nonrelativistic
and a relativistic two-nucleon Schr\"odinger equation is proposed which allows
to analytically rewrite the two Schr\"odinger equations into each other. As a
consequence a NN potential occurring in the relativistic Schr\"odinger equation
can be gained from a nonrelativistic one by an analytical procedure. The
S-matrices in the two equations are exactly identical and therefore the
two-nucleon phase shifts.
