theorem Th28:
  exp(2*PI*n*<i>) = 1
proof
  thus exp(2*PI*n*<i>) = cos(0+2*PI*n)+sin(0+2*PI*n)*<i> by SIN_COS:25
    .=cos.(0+2*PI*n)+sin(0+2*PI*n)*<i> by SIN_COS:def 19
    .=cos.(0+2*PI*n)+sin.(0+2*PI*n)*<i> by SIN_COS:def 17
    .=cos.(0+2*PI*n)+sin.(0)*<i> by SIN_COS2:10
    .= 1 by SIN_COS:30,SIN_COS2:11;
end;
