\begin{equation} \DeclareMathOperator\Gr{Gr} \DeclareMathOperator\LGr{LGr} \DeclareMathOperator\OGr{OGr} \DeclareMathOperator\SGr{SGr} \DeclareMathOperator\Kzero{K_0} \DeclareMathOperator\index{i} \DeclareMathOperator\rk{rk} \end{equation}

Grassmannian.info

A periodic table of (generalised) Grassmannians.

Odd symplectic Grassmannian $\operatorname{SGr}(2,15)=X^3(7, 2)$

Y Z Y Z Y Z Y Z Y Z Y Z Y Z
Betti numbers
\begin{align*} \mathrm{b}_{ 0 } &= 1 \\ \mathrm{b}_{ 2 } &= 1 \\ \mathrm{b}_{ 4 } &= 2 \\ \mathrm{b}_{ 6 } &= 2 \\ \mathrm{b}_{ 8 } &= 3 \\ \mathrm{b}_{ 10 } &= 3 \\ \mathrm{b}_{ 12 } &= 4 \\ \mathrm{b}_{ 14 } &= 4 \\ \mathrm{b}_{ 16 } &= 5 \\ \mathrm{b}_{ 18 } &= 5 \\ \mathrm{b}_{ 20 } &= 6 \\ \mathrm{b}_{ 22 } &= 6 \\ \mathrm{b}_{ 24 } &= 7 \\ \mathrm{b}_{ 26 } &= 7 \\ \mathrm{b}_{ 28 } &= 6 \\ \mathrm{b}_{ 30 } &= 6 \\ \mathrm{b}_{ 32 } &= 5 \\ \mathrm{b}_{ 34 } &= 5 \\ \mathrm{b}_{ 36 } &= 4 \\ \mathrm{b}_{ 38 } &= 4 \\ \mathrm{b}_{ 40 } &= 3 \\ \mathrm{b}_{ 42 } &= 3 \\ \mathrm{b}_{ 44 } &= 2 \\ \mathrm{b}_{ 46 } &= 2 \\ \mathrm{b}_{ 48 } &= 1 \\ \mathrm{b}_{ 50 } &= 1 \end{align*}
Basic information
dimension
25
index
14
Euler characteristic
98
Betti numbers
$\mathrm{b}_{ 0 } = 1$, $\mathrm{b}_{ 2 } = 1$, $\mathrm{b}_{ 4 } = 2$, $\mathrm{b}_{ 6 } = 2$, $\mathrm{b}_{ 8 } = 3$, $\mathrm{b}_{ 10 } = 3$, $\mathrm{b}_{ 12 } = 4$, $\mathrm{b}_{ 14 } = 4$, $\mathrm{b}_{ 16 } = 5$, $\mathrm{b}_{ 18 } = 5$, $\mathrm{b}_{ 20 } = 6$, $\mathrm{b}_{ 22 } = 6$, $\mathrm{b}_{ 24 } = 7$, $\mathrm{b}_{ 26 } = 7$, $\mathrm{b}_{ 28 } = 6$, $\mathrm{b}_{ 30 } = 6$, $\mathrm{b}_{ 32 } = 5$, $\mathrm{b}_{ 34 } = 5$, $\mathrm{b}_{ 36 } = 4$, $\mathrm{b}_{ 38 } = 4$, $\mathrm{b}_{ 40 } = 3$, $\mathrm{b}_{ 42 } = 3$, $\mathrm{b}_{ 44 } = 2$, $\mathrm{b}_{ 46 } = 2$, $\mathrm{b}_{ 48 } = 1$, $\mathrm{b}_{ 50 } = 1$
$G$
$\mathrm{PSp}_{ 14 }$
$\dim G$
98
$\mathrm{Aut}^0(X^3(7, 2))$
$G\ltimes U$
$\dim\mathrm{Aut}^0(X^3(7, 2))$
147
Geometric description
zero locus of $\bigwedge^2\mathcal{U}^\vee$ in $\operatorname{Gr}(2,15)$.
Blowups and projections
role dimension codimension index
$Z=\mathbb{P}^{13}$ Y Z Y Z Y Z Y Z Y Z Y Z Y Z unique closed $\mathrm{Aut}(X^3(7, 2))$-orbit 13 12 14
$Y=\SGr(2,14)$ Y Z Y Z Y Z Y Z Y Z Y Z Y Z other closed $\mathrm{PSp}_{ 14 }$ -orbit 23 2 13
\begin{equation} \xymatrix{ E_{ \mathbb{P}^{13} } \ar@{^{(}->}[r] \ar@{->>}[d] & \operatorname{Bl}_{ \mathbb{P}^{13} } X^3(7, 2) \ar@{->>}[r]^(.6){\mathbb{P}^{ 2 }} \ar@{->>}[d] & \href{/C7/2 }{ \SGr(2,14) } \\ \href{/C7/1 }{ \mathbb{P}^{13} } \ar[r] & X^3(7, 2) } \end{equation} \begin{equation} \xymatrix{ E_{ \SGr(2,14) } \ar@{^{(}->}[r] \ar@{->>}[d] & \operatorname{Bl}_{ \SGr(2,14) } X^3(7, 2) \ar@{->>}[r]^(.6){\mathbb{P}^{ 12 }} \ar@{->>}[d] & \href{/C7/1 }{ \mathbb{P}^{13} } \\ \href{/C7/2 }{ \SGr(2,14) } \ar[r] & X^3(7, 2) } \end{equation}
The exceptional divisor is the partial flag variety Y Z Y Z Y Z Y Z Y Z Y Z Y Z
$ E_{ \SGr(2,14) } \cong E_{ \mathbb{P}^{13} } \cong \mathrm{ C }_{ 7 } / \mathrm{P}_{ 2, 1 } $
Exceptional collections
  • Pech constructed a full exceptional sequence in 2013, see MR2998953 .
Quantum cohomology
small
generically semisimple
big
generically semisimple

Eigenvalues of quantum multiplication by $\mathrm{c}_1(X^3(7, 2))$:

Homological projective duality