Intersection number of immersions
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$$I_M=\lambda_M=\lambda: H_{n_1}(M)\times H_{n_2}(M) \to \Z$$ | $$I_M=\lambda_M=\lambda: H_{n_1}(M)\times H_{n_2}(M) \to \Z$$ | ||
the [[Intersection_form#Definition|''homology intersection pairing (or product)'']] of $M$. | the [[Intersection_form#Definition|''homology intersection pairing (or product)'']] of $M$. | ||
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+ | Let $N_i$ be oriented $n_i$-manifolds $(i=1,2)$. | ||
+ | The ''double point set'' of maps $f_i:N_i\to M$ $(i=1,2)$ is defined by | ||
+ | $$S_2(f_1,f_2) = \{(x_1,x_2)\in N_1\times N_2 | f_1(x_1) = f_2(x_2)\in M\} = | ||
+ | (f_1\times f_2)^{-1}(\Delta(M)),$$ | ||
+ | where $\Delta(M) = \{(x,x)\in M\times M | x\in M\}$ the diagonal. | ||
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+ | A double point $x=(x_1,x_2)\in S_2(f_1,f_2)$ of immersions $f_i:N_i\looparrowright M$ $(i=1,2)$ is ''transverse'' if the linear map | ||
+ | $$df(x) = df_1(x_1)\oplus df_2(x_2): \tau_{N_1}(x_1)\oplus \tau_{N_2}(x_2) \to \tau_M(f(x))$$ | ||
+ | is an isomorphism. | ||
+ | Immersions $f_i:N_i\looparrowright M$ $(i=1,2)$ have ''transverse intersection'' (or are ''transverse'') if $S_2(f_1,f_2)$ is finite and every double point is transverse. | ||
+ | |||
+ | The ''index'', or the ''sign'' $I(x)\in\Z$ of a transverse double point $x=(x_1,x_2)\in S_2(f_1,f_2)$ is | ||
+ | $$I(x) = \left\{ \begin{array}{cc} +1, & \mathrm{if}\; df(x)\; \mathrm{preserves}\; \mathrm{orientations}\\ -1, & \mathrm{otherwise}.\end{array}\right.$$ | ||
{{beginthm|Theorem}}\label{t:algeo} | {{beginthm|Theorem}}\label{t:algeo} | ||
− | For any immersions $f_1:N_1\looparrowright M$, $f_2:N_2\looparrowright M$ of oriented $n_1$- and $n_2$-manifolds | + | For any transverse immersions $f_1:N_1\looparrowright M$, $f_2:N_2\looparrowright M$ of oriented $n_1$- and $n_2$-manifolds |
+ | $$\lambda(f_{1*}[N_1],f_{2*}[N_2])=\sum_{x\in S_2(f_1,f_2)}{I(x)}.$$ | ||
{{endthm}} | {{endthm}} | ||
This this clasical fact is either a theorem or a definition depending on which definition of homology intersection pairing one accepts. | This this clasical fact is either a theorem or a definition depending on which definition of homology intersection pairing one accepts. | ||
For a proof see \cite{Scorpan2005|Section 3.2} or \cite{Ranicki2002|Proposition 7.22}. | For a proof see \cite{Scorpan2005|Section 3.2} or \cite{Ranicki2002|Proposition 7.22}. | ||
− | + | Unless this equality is a definition, the left- and right- hand sides of the equality can be called ''algebraic'' and ''geometric'' intersection number of $f_1$ and $f_2$. | |
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+ | $\lambda^{\mathrm{geo}}(N_1,N_2)$ defined below. | ||
$\lambda^{\mathrm{alg}}(f_1,f_2)\in\Z$ $$\lambda^{\mathrm{alg}}(N_1,N_2) := \lambda((f_1)_*[N_1],(f_2)_*[N_2]).$$ | $\lambda^{\mathrm{alg}}(f_1,f_2)\in\Z$ $$\lambda^{\mathrm{alg}}(N_1,N_2) := \lambda((f_1)_*[N_1],(f_2)_*[N_2]).$$ | ||
the homology intersection of the homology classes $(f_1)_*[N_1]\in H_{n_1}(M)$, $(f_2)_*[N_2]\in H_{n_2}(M)$: | the homology intersection of the homology classes $(f_1)_*[N_1]\in H_{n_1}(M)$, $(f_2)_*[N_2]\in H_{n_2}(M)$: | ||
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==Alternative description== | ==Alternative description== | ||
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The ''geometric'' intersection number of transverse immersions $f_i:N_i\looparrowright M$ $(i=1,2)$ is defined as | The ''geometric'' intersection number of transverse immersions $f_i:N_i\looparrowright M$ $(i=1,2)$ is defined as | ||
$$\lambda^{\mathrm{geo}}(N_1,N_2) = \sum_{x\in S_2(f_1,f_2)}{I(x)}\in \Z.$$ | $$\lambda^{\mathrm{geo}}(N_1,N_2) = \sum_{x\in S_2(f_1,f_2)}{I(x)}\in \Z.$$ | ||
− | + | The algebraic and geometric intersection numbers agree, | |
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$$\lambda^{\mathrm{alg}}(N_1,N_2)=\lambda^{\mathrm{geo}}(N_1,N_2).$$ --> | $$\lambda^{\mathrm{alg}}(N_1,N_2)=\lambda^{\mathrm{geo}}(N_1,N_2).$$ --> | ||
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==References== | ==References== |
Revision as of 14:52, 2 April 2019
This page has not been refereed. The information given here might be incomplete or provisional. |
1 Introduction
This page is based on [Ranicki2002], see also [Broecker&Jaenich1982, Excercise 14.9.6]. Let , be immersions of oriented manifolds in a connected oriented manifold. The intersection number has both an algebraic and geometric formulation; roughly speaking it counts with sign the number of intersection points that the two immersions have. The intersection number is an obstruction to perturbing the immersions into being disjoint. When it vanishes this perturbation can often be achieved using the Whitney trick. The intersection number of immersions is closely related to the intersection form of a manifold (and so to the signature of a -dimensional manifold) and to characteristic classes. These are important invariants used in the classification of manifolds.
2 Statement
Let be a connected oriented manifold of dimension and
the homology intersection pairing (or product) of .
Let be oriented -manifolds . The double point set of maps is defined by
where the diagonal.
A double point of immersions is transverse if the linear map
is an isomorphism. Immersions have transverse intersection (or are transverse) if is finite and every double point is transverse.
The index, or the sign of a transverse double point is
Theorem 2.1. For any transverse immersions , of oriented - and -manifolds
This this clasical fact is either a theorem or a definition depending on which definition of homology intersection pairing one accepts. For a proof see [Scorpan2005, Section 3.2] or [Ranicki2002, Proposition 7.22]. Unless this equality is a definition, the left- and right- hand sides of the equality can be called algebraic and geometric intersection number of and .
References
- [Broecker&Jaenich1982] Th. Br\"ocker and K. J\"anich Introduction to Differential Topology, Cambridge University Press, 1982. ISBN-13: 978-0521284707, ISBN-10: 0521284708.
- [Ranicki2002] A. Ranicki, Algebraic and geometric surgery, The Clarendon Press Oxford University Press, Oxford, 2002. MR2061749 (2005e:57075) Zbl 1003.57001
- [Scorpan2005] A. Scorpan, The wild world of 4-manifolds, American Mathematical Society, 2005. MR2136212 (2006h:57018) Zbl 1075.57001