Skip to main content
replaced http://mathoverflow.net/ with https://mathoverflow.net/
Source Link

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)$ and $N_D$, the normal bundle of $D$ in $X$. To get $H^i(X,T_X(-\log D))$ one can use the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$or $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$These sequences make sense even when $D$ is singular (see Sernesi's book on deformation theory). These sheaves and their cohomology groups can be handeled in, say, Macaulay2, if you have the explicit equations.

EDIT: Let me try to explain the above in more detail: If one is in the lucky position that some of the cohomology groups of, say $N_D$, vanish (e.g., a line $L$ on a cubic surface has $h^0(L,N_L)=h^1(L,N_L)=0$, you can calculate the cohomology almost immediately. In the general case, you will have to write down the maps and calculate the cohomology using say, Cech cohomology. Macaulay2 have built-in routines for this, see for example Francesco Polizzi's answer to this questionthis question.

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)$ and $N_D$, the normal bundle of $D$ in $X$. To get $H^i(X,T_X(-\log D))$ one can use the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$or $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$These sequences make sense even when $D$ is singular (see Sernesi's book on deformation theory). These sheaves and their cohomology groups can be handeled in, say, Macaulay2, if you have the explicit equations.

EDIT: Let me try to explain the above in more detail: If one is in the lucky position that some of the cohomology groups of, say $N_D$, vanish (e.g., a line $L$ on a cubic surface has $h^0(L,N_L)=h^1(L,N_L)=0$, you can calculate the cohomology almost immediately. In the general case, you will have to write down the maps and calculate the cohomology using say, Cech cohomology. Macaulay2 have built-in routines for this, see for example Francesco Polizzi's answer to this question.

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)$ and $N_D$, the normal bundle of $D$ in $X$. To get $H^i(X,T_X(-\log D))$ one can use the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$or $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$These sequences make sense even when $D$ is singular (see Sernesi's book on deformation theory). These sheaves and their cohomology groups can be handeled in, say, Macaulay2, if you have the explicit equations.

EDIT: Let me try to explain the above in more detail: If one is in the lucky position that some of the cohomology groups of, say $N_D$, vanish (e.g., a line $L$ on a cubic surface has $h^0(L,N_L)=h^1(L,N_L)=0$, you can calculate the cohomology almost immediately. In the general case, you will have to write down the maps and calculate the cohomology using say, Cech cohomology. Macaulay2 have built-in routines for this, see for example Francesco Polizzi's answer to this question.

added 678 characters in body
Source Link
J.C. Ottem
  • 11.6k
  • 2
  • 42
  • 79

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)=T_X\otimes I_D$ $T_X, \quad T_X(-D)$ and $N_D$, the normal bundle of $D$ in $X$. To get $H^i(X,T_X(-\log D))$ one can use the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$or $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$These sequences make sense even when $D$ is singular (see Sernesi's book on deformation theory). These sheaves and their cohomology groups can be calculatedhandeled in, say, Macaulay2, if you have the explicit equations.

To get $H^i(X,T_X(-\log D))$EDIT: Let me try to explain the above in more detail: If one can then takeis in the lucky position that some of the cohomology groups of, say $N_D$, vanish (e.g., a line $L$ on a cubic surface has $h^0(L,N_L)=h^1(L,N_L)=0$, you can calculate the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$and $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$cohomology almost immediately. In the general case, you will have to write down the maps and calculate the cohomology using say, Cech cohomology. Macaulay2 have built-in routines for this, see for example Francesco Polizzi's answer to this question.

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)=T_X\otimes I_D$ and $N_D$, the normal bundle of $D$ in $X$. These cohomology groups can be calculated in, say, Macaulay2, if you have the explicit equations.

To get $H^i(X,T_X(-\log D))$ one can then take cohomology of the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$and $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)$ and $N_D$, the normal bundle of $D$ in $X$. To get $H^i(X,T_X(-\log D))$ one can use the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$or $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$These sequences make sense even when $D$ is singular (see Sernesi's book on deformation theory). These sheaves and their cohomology groups can be handeled in, say, Macaulay2, if you have the explicit equations.

EDIT: Let me try to explain the above in more detail: If one is in the lucky position that some of the cohomology groups of, say $N_D$, vanish (e.g., a line $L$ on a cubic surface has $h^0(L,N_L)=h^1(L,N_L)=0$, you can calculate the cohomology almost immediately. In the general case, you will have to write down the maps and calculate the cohomology using say, Cech cohomology. Macaulay2 have built-in routines for this, see for example Francesco Polizzi's answer to this question.

Source Link
J.C. Ottem
  • 11.6k
  • 2
  • 42
  • 79

Computing cohomology of the logarithmic tangent sheaf $T_X(-\log D)$ is a usually easier once you know the cohomology of the sheaves $T_X, \quad T_X(-D)=T_X\otimes I_D$ and $N_D$, the normal bundle of $D$ in $X$. These cohomology groups can be calculated in, say, Macaulay2, if you have the explicit equations.

To get $H^i(X,T_X(-\log D))$ one can then take cohomology of the exact sequences $$ 0 \to T_X(-D) \to T_X(-\log D) \to T_D \to 0 $$and $$ 0 \to T_X(-\log D) \to TX\to N_{D}\to 0. $$