Theorem^[1] (Banach) — If ${\displaystyle L:X\to Y}$ is a continuous linear map between two Fréchet spaces, then ${\displaystyle L:X\to Y}$ is surjective if and only if the following two conditions both hold:

1. ${\displaystyle {}^{t}L:Y^{\prime }\to X^{\prime }}$ is injective, and
2. the image of ${\displaystyle {}^{t}L,}$ denoted by ${\displaystyle \operatorname {Im} {}^{t}L,}$ is weakly closed in ${\displaystyle X^{\prime }}$ (i.e. closed when ${\displaystyle X^{\prime }}$ is endowed with the weak-* topology).

Theorem^[1] — If ${\displaystyle L:X\to Y}$ is a continuous linear map between two Fréchet spaces then the following are equivalent:

1. ${\displaystyle L:X\to Y}$ is surjective.
2. The following two conditions hold:
   1. ${\displaystyle {}^{t}L:Y^{\prime }\to X^{\prime }}$ is injective;
   2. the image ${\displaystyle \operatorname {Im} {}^{t}L}$ of ${\displaystyle {}^{t}L}$ is weakly closed in ${\displaystyle X^{\prime }.}$
3. For every continuous seminorm ${\displaystyle p}$ on ${\displaystyle X}$ there exists a continuous seminorm ${\displaystyle q}$ on ${\displaystyle Y}$ such that the following are true:
   1. for every ${\displaystyle y\in Y}$ there exists some ${\displaystyle x\in X}$ such that ${\displaystyle q(L(x)-y)=0}$;
   2. for every ${\displaystyle y^{\prime }\in Y,}$ if ${\displaystyle {}^{t}L\left(y^{\prime }\right)\in X_{p}^{\prime }}$ then ${\displaystyle y^{\prime }\in Y_{q}^{\prime }.}$
4. For every continuous seminorm ${\displaystyle p}$ on ${\displaystyle X}$ there exists a linear subspace ${\displaystyle N}$ of ${\displaystyle Y}$ such that the following are true:
   1. for every ${\displaystyle y\in Y}$ there exists some ${\displaystyle x\in X}$ such that ${\displaystyle L(x)-y\in N}$;
   2. for every ${\displaystyle y^{\prime }\in Y^{\prime },}$ if ${\displaystyle {}^{t}L\left(y^{\prime }\right)\in X_{p}^{\prime }}$ then ${\displaystyle y^{\prime }\in N^{\circ }.}$
5. There is a non-increasing sequence ${\displaystyle N_{1}\supseteq N_{2}\supseteq N_{3}\supseteq \cdots }$ of closed linear subspaces of ${\displaystyle Y}$ whose intersection is equal to ${\displaystyle \{0\}}$ and such that the following are true:
   1. for every ${\displaystyle y\in Y}$ and every positive integer ${\displaystyle k}$, there exists some ${\displaystyle x\in X}$ such that ${\displaystyle L(x)-y\in N_{k}}$;
   2. for every continuous seminorm ${\displaystyle p}$ on ${\displaystyle X}$ there exists an integer ${\displaystyle k}$ such that any ${\displaystyle x\in X}$ that satisfies ${\displaystyle L(x)\in N_{k}}$ is the limit, in the sense of the seminorm ${\displaystyle p}$, of a sequence ${\displaystyle x_{1},x_{2},\ldots }$ in elements of ${\displaystyle X}$ such that ${\displaystyle L\left(x_{i}\right)=0}$ for all ${\displaystyle i.}$

Theorem^[1] — Let ${\displaystyle X}$ be a Fréchet space and ${\displaystyle Z}$ be a linear subspace of ${\displaystyle X^{\prime }.}$ The following are equivalent:

1. ${\displaystyle Z}$ is weakly closed in ${\displaystyle X^{\prime }}$;
2. There exists a basis ${\displaystyle {\mathcal {B}}}$ of neighborhoods of the origin of ${\displaystyle X}$ such that for every ${\displaystyle B\in {\mathcal {B}},}$ ${\displaystyle B^{\circ }\cap Z}$ is weakly closed;
3. The intersection of ${\displaystyle Z}$ with every equicontinuous subset ${\displaystyle E}$ of ${\displaystyle X^{\prime }}$ is relatively closed in ${\displaystyle E}$ (where ${\displaystyle X^{\prime }}$ is given the weak topology induced by ${\displaystyle X}$ and ${\displaystyle E}$ is given the subspace topology induced by ${\displaystyle X^{\prime }}$).

Theorem^[1] — On the dual ${\displaystyle X^{\prime }}$ of a Fréchet space ${\displaystyle X}$, the topology of uniform convergence on compact convex subsets of ${\displaystyle X}$ is identical to the topology of uniform convergence on compact subsets of ${\displaystyle X}$.

Theorem^[1] — Let ${\displaystyle L:X\to Y}$ be a linear map between Hausdorff locally convex TVSs, with ${\displaystyle X}$ also metrizable. If the map ${\displaystyle L:\left(X,\sigma \left(X,X^{\prime }\right)\right)\to \left(Y,\sigma \left(Y,Y^{\prime }\right)\right)}$ is continuous then ${\displaystyle L:X\to Y}$ is continuous (where ${\displaystyle X}$ and ${\displaystyle Y}$ carry their original topologies).

Theorem^[2] (E. Borel) — Fix a positive integer ${\displaystyle n}$. If ${\displaystyle P}$ is an arbitrary formal power series in ${\displaystyle n}$ indeterminates with complex coefficients then there exists a ${\displaystyle {\mathcal {C}}^{\infty }}$ function ${\displaystyle f:\mathbb {R} ^{n}\to \mathbb {C} }$ whose Taylor expansion at the origin is identical to ${\displaystyle P}$.

That is, suppose that for every ${\displaystyle n}$-tuple of non-negative integers ${\displaystyle p=\left(p_{1},\ldots ,p_{n}\right)}$ we are given a complex number ${\displaystyle a_{p}}$ (with no restrictions). Then there exists a ${\displaystyle {\mathcal {C}}^{\infty }}$ function ${\displaystyle f:\mathbb {R} ^{n}\to \mathbb {C} }$ such that ${\displaystyle a_{p}=\left(\partial /\partial x\right)^{p}f{\bigg \vert }_{x=0}}$ for every ${\displaystyle n}$-tuple ${\displaystyle p.}$

Theorem^[3] — Let ${\displaystyle D}$ be a linear partial differential operator with ${\displaystyle {\mathcal {C}}^{\infty }}$ coefficients in an open subset ${\displaystyle U\subseteq \mathbb {R} ^{n}.}$ The following are equivalent:

1. For every ${\displaystyle f\in {\mathcal {C}}^{\infty }(U)}$ there exists some ${\displaystyle u\in {\mathcal {C}}^{\infty }(U)}$ such that ${\displaystyle Du=f.}$
2. ${\displaystyle U}$ is ${\displaystyle D}$-convex and ${\displaystyle D}$ is semiglobally solvable.