The equation should be
W ( x 1 , x 2 ) ( t ) = x 1 ( t ) x ˙ 2 ( t ) − x 2 ( t ) x ˙ 1 ( t ) ≠ 0 {\displaystyle W(x_{1},x_{2})(t)=x_{1}(t){\dot {x}}_{2}(t)-x_{2}(t){\dot {x}}_{1}(t)\neq 0}
Return to errata.
