In this section, our aim is to prove several closely related results, all of which are occasionally called "Picard-Lindelöf theorem". This type of result is often used when it comes to arguing for the existence and uniqueness of a certain ordinary differential equation, given that some boundary conditions are satisfied.
Picard–Lindelöf Theorem (Banach fixed-point theorem version):
Let
be an interval, let
be a continuous function, and let

be the associated ordinary differential equation. If
is Lipschitz continuous in the second argument, then this ODE possesses a unique solution on
for each possible initial value
, where
,
being the Lipschitz constant of the second argument of
.
Proof:
We first rewrite the problem as a fixed-point problem. Indeed, using the fundamental theorem of calculus, one can show that the simultaneous equations
![{\displaystyle {\begin{cases}x'(t)=f(t,x(t))&t\in [a,a+\epsilon ]\\x(0)=x_{0}&\end{cases}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/eb9f8fa714aab7dd19a06498f8e93da2899b40ed)
are equivalent to the single equation
,
where
is to be determined at a later stage. This means that the function
is a fixed point of the function
.
Now
satisfies a Lipschitz condition as follows:

where we took the norm on
to be the supremum norm. If now
, then
is a contraction, and hence the Banach fixed-point theorem is applicable, giving us both existence and uniqueness.
Replacing the fixed-point principle by summation techniques, we get a slightly better result in the sense that the domain of definition of the function
does not have to be all of
.
Picard–Lindelöf theorem (telescopic series version):
Let
be a function which is continuous and Lipschitz continuous in the second argument, where
, and let
with the property that
for some
. If in this case
, where
, then the initial value problem
![{\displaystyle {\begin{cases}x'(t)=f(t,x(t))&t\in [t_{0}-\gamma ,t_{0}+\gamma ]\\x(t_{0})=x_{0}&\end{cases}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7de426e603c333c9ebb0fef370fda9b445b0cecf)
possesses a unique solution.
Proof:
We first prove uniqueness. To do so, we use Gronwall's inequalities. Suppose
are both solutions to the problem. Then
,
and hence by Gronwall's inequalities

for both
(right Gronwall's inequality) and
(left Gronwall's inequality).
Now on to existence. Once again, we inductively define
(the constant function),
.
Since
is not necessarily defined on any larger set than
, we have to prove that this definition always makes sense, i.e. that
is defined for all
and
, that is,
for
. We prove this by induction.
For
, this is trivial.
Assume now that
for
. Then

For
we obtain an analogous bound.
By the telescopic sum, we have
.
Furthermore, for
and
,

Hence, by induction,
.
Again, by the very same argument, an analogous bound holds for
.
Thus, by the Weierstraß M-test, the telescopic sum

converges uniformly; in particular,
converges.
It is now possible to interchange differentiation and summation in the latter sum; for, on the one hand, we are uniformly convergent, and on the other hand,
,
which converges to
for
due to theorem 2.5 and the convergence of
; note that the image of each
is contained within the compact set
, the closure of
. Hence indeed

on
.