Let $G$ be a compact connected and simply connected Lie group and $G^\mathbb{C}$ be the complexification of Lie group (with is diffeomorphic with $G^\mathbb{C}\cong T^*G$) then I am looking for finding the 2nd homotopy group $\pi_2(G^\mathbb{C}/P)$ where $P$ here is Parabolic subgroup. Is there any method or referrence ?

1$\begingroup$ Why do you call $\pi_2$ the "fundamental group"? $\endgroup$ – Jason Starr Feb 6 '14 at 20:08

$\begingroup$ You use the homotopy exact sequence, and you know that $G/P$ is connected and simply connected. You know the fundamental group of $G$ (see Serre, Complex Semisimple Lie Algebras). That should get you started, but I can't remember if I ever worked out the answer to this question. $\endgroup$ – Ben McKay Feb 6 '14 at 20:18

1$\begingroup$ There is a plenty of material on cohomology of generalizaed flag varieties. The cohomology of generalized flag varieties with coefficients in $\mathbb{C}$ is basically computed by Kostant's theorem from 62. $\endgroup$ – Vít Tuček Feb 6 '14 at 21:54

2$\begingroup$ en.wikipedia.org/wiki/Bruhat_decomposition $H_2 = \mathbb Z^{rank(G)rank(P)}$ This is not a researchlevel question. $\endgroup$ – Allen Knutson Feb 7 '14 at 3:21

1$\begingroup$ Yes, as is directly derived from the Bruhat decomposition of $G^{\mathbb C}/P$. $\endgroup$ – Allen Knutson Feb 8 '14 at 5:36
As @abx has indicated, $\pi_2(G_{\mathbb{C}}/P)$ is isomorphic to $H_2(G_{\mathbb{C}}/P)$. To describe the latter group, we use the Bruhat decomposition of $G_{\mathbb{C}}/P$. Choose a maximal torus $T$ and Borel $B$ satisfying $T\subseteq B\subseteq P$. We then have the Bruhat decomposition $$G_{\mathbb{C}}/P=\coprod_{[w]\in W/W_P}BwP/P,$$ where $W_P$ is the subgroup of $W$ generated by the reflections for the simple roots whose negative root spaces belong to the Lie algebra of $P$. The dimension of the Bruhat cell indexed by a coset in $W/W_P$ is twice the length of the coset representative of minimal length. So, all cells are evendimensional, and $H_2(G_{\mathbb{C}}/P)$ is the free abelian group of rank equal to the number of cosets in $W/W_P$ with minimal representatives of length $1$. So, this rank is equal to the number of simple roots whose negative root spaces are not in the Lie algebra of $P$. (If $P=B$, then this rank is precisely the number of simple roots.) I hope this is the sort of answer you are seeking.