El índice de Fredholm generaliza el teorema de rango-nulidad a espacios de dimensión infinita como los espacios de Banach. Para un operador de Fredholm [latex]T: X to Y[/latex], su índice se define como [latex]text{ind}(T) = dim(ker(T)) – dim(text{coker}(T))[/latex], donde la dimensión del Cokernel’s mide qué tan lejos está la imagen de ser todo el espacio. Este índice es un valor entero estable bajo pequeñas perturbaciones del operador.
El rank-nullity theorem, [latex]\dim(V) – \text{rank}(T) = \text{nullity}(T)[/latex], holds for linear maps between finite-dimensional vector spaces. In this context, [latex]\dim(V) – \text{rank}(T)[/latex] is the dimension of the cokernel, [latex]\text{coker}(T) = W/\text{im}(T)[/latex]. Thus, the theorem can be written as [latex]\dim(\ker(T)) – \dim(\text{coker}(T)) = 0[/latex]. The Fredholm index extends this idea to Fredholm operators, which are bounded linear operators between Banach spaces whose kernel and cokernel are both finite-dimensional.
Para un operador de este tipo [latex]T: X to Y[/latex], el índice de Fredholm es [latex]text{ind}(T) = dim(ker(T)) – dim(text{coker}(T))[/latex]. A diferencia del caso de dimensión finita, donde esta diferencia siempre es cero, para espacios de dimensión infinita, el índice puede ser cualquier entero. Una propiedad clave del índice es su estabilidad: no cambia bajo perturbaciones compactas del operador. Esto significa que si [latex]K[/latex] es un operador compacto, entonces [latex]text{ind}(T+K) = text{ind}(T)[/latex].
The concept of the cokernel is crucial for this generalization. For a map [latex]T: X \to Y[/latex], the image [latex]\text{im}(T)[/latex] is a subspace of the codomain [latex]Y[/latex]. The cokernel, [latex]\text{coker}(T)[/latex], is the quotient space [latex]Y / \text{im}(T)[/latex]. Its dimension measures the ‘number of independent directions’ in [latex]Y[/latex] that are not reached by [latex]T[/latex]. In finite dimensions, the teorema de rango-nulidad implies [latex]\dim(\ker(T)) = \dim(\text{coker}(T))[/latex]. In infinite dimensions, this equality breaks down, but the difference between these two finite dimensions remains a stable integer, the Fredholm index.
This stability makes the index a powerful topological invariant. It plays a central role in the Atiyah-Singer index theorem, one of the most profound results of 20th-century mathematics, which connects the analytical index of a differential operator on a compact manifold to topological invariants of that manifold. This bridges the gap between analysis and topology, with far-reaching consequences in theoretical physics and geometry.
Relacionado con: índice de Fredholm, análisis funcional, espacio de Banach, operador de Fredholm, núcleo, conúcleo, teorema del índice de Atiyah-Singer, teoría de operadores, invariante topológico, dimensión infinita.