This question has some overlap with previous ones but doesn't seem to have a well-documented answer. I recall some literature (mostly involving Lie groups and hermitian symmetric pairs, etc.) which concerns *maximal* parabolic subalgebras of a simple Lie algebra $\mathfrak{g}$ over the field $\mathbb{C}$ or related parabolic subgroups of Lie groups. Here $\mathfrak{p}$ can be taken as standard relative to some fixed Borel subalgebra and Cartan subalgebra, with a decomposition as direct sum of a Levi subalgebra (involving all but one simple root) along with the nilradical $\mathfrak{n}$. 



> 
> In this situation, is there a necessary and sufficient criterion in the literature for $\mathfrak{n}$ to be abelian, which can then be checked easily case-by-case for the simple types?
> 
> 
> 


For example, neither of the two types of maximal (=minimal) parabolic subalgebra in the Lie algebra $G\_2$ turns out to have an abelian nilradical. It's also true that $G\_2$ has no *minuscule* highest weights for its irreducible finite dimensional representations. 


On the other hand, one of the equivalent conditions for a dominant integral weight to be (co)minuscule implies that the nilradical of the parabolic subalgebra stabilizing a highest weight vector in the corresponding representation must be abelian. (See my previous question [here](https://mathoverflow.net/questions/108468/) for some references on minuscule weights.) But I don't recall now exactly how precise a criterion exists in the literature for $\mathfrak{n}$ to be abelian when $\mathfrak{p}$ is maximal. 


[I was just thinking about this in connection with a newer question [here](https://mathoverflow.net/questions/129014/) which is not precisely
formulated but apparently involves a similar setting.]


ADDED: The answer (plus email) and references given are quite helpful though somewhat intertwined with Lie groups and differential geometry or algebraic groups and algebraic geometry. I was looking for a self-contained approach via roots and weights within the traditional Lie algebra setting. Anyway, a uniform elementary statement seems to emerge as follows. With $\mathfrak{g}$ simple, take $\mathfrak{p}$ to be a maximal parabolic corresponding to the set of simple roots excluding $\alpha$. Then $\mathfrak{n}$ is abelian iff $\alpha$ has coefficient 1 in the highest root, or iff $\mathfrak{p}$ is the stabilizer of a highest weight vector in the irreducible representation whose highest weight is "cominuscule" relative to $\alpha$ (involving interchange of types B, C). (These criteria are then easy to check case-by-case.)



Criterion for nilradical of a maximal parabolic subalgebra to be abelian?
Denote by $\mathfrak{l}$ the Levi factor of the parabolic, so that $\mathfrak{p} = \mathfrak{l} \oplus \mathfrak{n}$, and note that this is a splitting as $\mathfrak{l}$-modules. Also denote by $\mathfrak{n}\_-$ the nilradical of the opposite parabolic subalgebra; this is the dual of $\mathfrak{n}$ via the Killing form of $\mathfrak{g}$.


Here are the equivalent conditions that I know:


1. $\mathfrak{n}$ is abelian.
2. $\mathfrak{n}\_-$ is abelian.
3. $\mathfrak{n}$ is an irreducible representation of $\mathfrak{l}$.
4. $\mathfrak{n}\_-$ is an irreducible representation of $\mathfrak{l}$.
5. $\mathfrak{p}$ is maximal and the simple root of $\mathfrak{g}$ that is removed from $\mathfrak{l}$ has coefficient 1 in the highest root of $\mathfrak{g}$.
6. $[\mathfrak{n},\mathfrak{n}] \subseteq \mathfrak{l}$.
7. $(\mathfrak{g},\mathfrak{l})$ is a symmetric pair, i.e. there is an involutive Lie algebra automorphism of $\mathfrak{g}$ whose fixed-point subalgebra is precisely $\mathfrak{l}$.


Clearly condition 5 is the easiest way to check this, assuming you have handy a table of highest roots of the simple Lie algebras. One can be found in, e.g. Table 2 in the exercises of Chapter 12 of *Introduction to Lie Algebras and Representation Theory*, by... well, you.


I have not seen this entire collection of equivalent criteria written up in one place, although many of them are discussed in Lemma 7.3.1 of *Multiplicity-free Theorems of the Restrictions of Unitary Highest Weight Modules with respect to Reductive Symmetric Pairs*, by Toshi Kobayashi.


