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Regular singular point of non-linear ODE: $dot{x}(t) + t^{-1}Ax(t) = Q(x(t))$

MathOverflow Asked on November 24, 2021

Consider a system of ordinary differential equations of the form
$$
dot{x}(t) + frac{1}{t}Ax(t) = Q(x(t))
$$

where $x(t) in mathbb{C}^n$, $A in mathrm{Mat}_{ntimes n}(mathbb{C})$ is a constant matrix, and $Q: mathbb{C}^n to mathbb{C}^n$ is homogeneous of degree $2$, i.e. $Q(lambda x) = lambda^2 Q(x)$ for $lambda in mathbb{C}$.

What is known about existence of solutions near $t = 0$?

If it were not for the quadratic term $Q$, the point $t = 0$ would be a regular singular point of the ODE and then we could use the Frobenius method. But in all the references I know, regular singular points are only discussed for linear systems.

One Answer

There's nothing inherently linear about constructing power series solutions à la Frobenius. The existence and uniqueness theory for a class of singular non-linear ODEs, of which yours is a special case, is treated for instance in Ch.IX of

Wasow, W., Asymptotic expansions for ordinary differential equations, (Dover, 1987) reprint from 1965. ZBL0169.10903

Answered by Igor Khavkine on November 24, 2021

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