Atiyah–Singer index theorem

The Atiyah–Singer index theorem is an important idea in mathematics, especially in differential geometry. It was proven by two mathematicians, Michael Atiyah and Isadore Singer, in 1963.

This theorem helps us understand a special connection between two different kinds of information about shapes.

One kind of information comes from solving certain math problems related to these shapes, called elliptic differential operators. The other kind of information comes from the shape’s topology, which is about how the shape is built and connected.

This theorem shows that these two very different kinds of information are actually the same for certain shapes, called compact manifolds. It includes many older theorems, like the Chern–Gauss–Bonnet theorem and the Riemann–Roch theorem, as special examples. Because of this, the Atiyah–Singer index theorem is also useful in areas like theoretical physics.

History

The idea behind the Atiyah–Singer index theorem began when Israel Gel'fand asked how to understand something called the "index" of special math rules known as elliptic differential operators. He noticed that this index stayed the same even when the shapes changed slightly.

Later, Michael Atiyah and Isadore Singer announced their important theorem in 1963. They showed that two different ways to calculate the index actually give the same answer. Since then, many mathematicians have used and expanded this work, finding new proofs and uses for the theorem in different areas of math.

Notation

In this topic, X represents a special kind of smooth shape called a compact smooth manifold without any edges. We also have E and F, which are smooth collections of directions called vector bundles placed over X. There is an operation called D, which is a special type of rule that moves smooth parts of E to smooth parts of F.

Symbol of a differential operator

A differential operator is a math tool we use to study how things change. The "symbol" of a differential operator is a special function that helps us understand it better.

For an operator of order n, the symbol is made by looking at the highest-order terms and replacing some parts with new variables.

For example, the Laplace operator measures how curved a surface is. Its symbol is never zero when at least one of its variables is not zero. This makes it an elliptic operator. But the wave operator, which describes waves, is not elliptic in more than one dimension because its symbol can be zero for some non-zero values.

On more complex spaces, the symbol is defined in a similar way and helps us know if the operator is elliptic. Elliptic operators are important because they have useful properties, like having a limited number of solutions.

Analytical index

A special math rule called an elliptic differential operator works like a puzzle solver. It can have answers (called the kernel) and rules about what answers are allowed (called the cokernel). The analytical index is the difference between the number of answers and the number of rules.

For example, think of a simple loop (like a circle) and a basic rule called D. Sometimes this rule has answers, and sometimes it doesn’t — it depends on a special number λ. But the difference between the number of answers and the number of rules always stays the same. This shows that even when things change, some important parts stay balanced and can be described using other math ideas.

Topological index

The topological index is a way to describe some math ideas using shapes and spaces. It helps connect two kinds of information: one from solving equations and one from the shape of the space.

This idea is important because it shows how solving hard problems can be easier by looking at the space in a new way. It also helps us learn deep facts about shapes and how they act.

Extensions of the Atiyah–Singer index theorem

Teleman index theorem

Teleman showed that for a special kind of math problem on a special kind of shape, two important numbers are the same.

Connes–Donaldson–Sullivan–Teleman index theorem

Donaldson, Sullivan, Connes, and Teleman found that for another kind of shape, there is a way to build special math classes using a special math operator.

Other extensions

The Atiyah–Singer theorem also works for other kinds of math problems and shapes. It can be used when the shape has edges, when looking at families of problems, or when a math group acts on the shape. It can even be used for very large shapes in a special way.

Examples

Chern-Gauss-Bonnet theorem

The Chern-Gauss-Bonnet theorem is a special case of the Atiyah–Singer index theorem. It connects a geometric idea (the Euler characteristic) with a topological idea (the Euler class).

For a special kind of mathematical space called a compact oriented manifold, the theorem says that a certain calculation involving shapes and angles will give the same result as a calculation using only the manifold’s topological properties.

Hirzebruch signature theorem

The Hirzebruch signature theorem is another special case of the Atiyah–Singer index theorem. It tells us that for certain special spaces of even dimension, a topological number called the signature can be calculated using another topological idea called the L genus.

 genus and Rochlin's theorem

The  genus is a number that can be calculated for any manifold. For special types of manifolds called spin manifolds, this number is always a whole number, and in certain dimensions, it is always an even whole number.

In four dimensions, this leads to Rochlin’s theorem, which says that the signature of a four-dimensional spin manifold is always divisible by 16.

Proof techniques

The Atiyah–Singer index theorem was proven using different methods. One method uses special math tools called pseudodifferential operators. These tools help mathematicians understand complex problems.

Another method uses ideas from geometry and topology, called cobordism theory. This method checks the theorem on simple cases and then shows it works for more complicated situations.

A later proof used the heat equation, a math tool that helps understand how things change over time. This proof shows that the theorem can be understood by looking at how certain math expressions behave when time gets very small.