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Calculus Level 3

Can graphs of two solutions to the equation y + x y = x 1 + x 2 y' + xy = \frac{x}{1+{x}^{2}} intersect at some point ( x 0 , y 0 ) ({x}_{0}, {y}_{0}) ?

No Yes

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2 solutions

Mark Hennings
Apr 29, 2021

The differential equation becomes d d x ( e 1 2 x 2 u ) = x 1 + x e 1 2 x 2 \frac{d}{dx}\big(e^{\frac12x^2}u\big) \; = \; \frac{x}{1+x}e^{\frac12x^2} and hence the general solution is y A ( x ) = A e 1 2 x 2 + e 1 2 x 2 0 x 2 e u 1 + u d u y_A(x) \; = \; Ae^{-\frac12x^2} + e^{-\frac12x^2}\int_0^{x^2}\frac{e^u}{1+u}\,du for some constant A A . If y A ( x 0 ) = y B ( x 0 ) y_A(x_0) = y_B(x_0) , then e 1 2 x 0 2 A = e 1 2 x 0 2 B e^{-\frac12x_0^2}A \; = \; e^{-\frac12x_0^2}B and hence A = B A = B , so y A y B y_A \equiv y_B . Thus two distinct solutions of this differential equation cannot meet at a point.

Doug Brunson
Apr 29, 2021

The differential operator satisfies the uniqueness requirement of Picard's theorem for all initial conditions.

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