2udu/( 1-u^2) = dy/y

-ln( 1-u^2) = lny+lnC

Cy( 1-u^2) = 1

Y (0) = 1，P (0) = Y' (0) =- 1，U (0) = P (0)/Y (0) =- 1？ C is infinite.

So only the constant function u = 1 and u(0) =-1 is satisfied, then u =-1.

p = -y，dy/dx = -y，dy/y = -dx

Lny =-x+lnc 1, y = c 1e (-x), y (0) = 1, C 1 = 1, then y = e (-x).