Construct an example of a first-order differential equation on

Mara Cook

Mara Cook

Answered question

2022-06-22

Construct an example of a first-order differential equation on R for which there are no solutions to any initial value problem.
Could anyone please get me started on this. I am struck as to which direction to go

Answer & Explanation

Kaydence Washington

Kaydence Washington

Beginner2022-06-23Added 32 answers

Simply consider x ˙ = f ( t ) where f is the Dirichlet function.
There's no solution for any initial value since f is discontinuous everywhere, while the derivative of a differentiable function should be continuous somewhere.
It's a direct consequence of Baire category theorem, since for any differentiable function g, g ( t ) = lim n n ( g ( t + 1 / n ) g ( t ) ), a pointwise limit of a sequence of continuous functions.

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