Lomonosov's invariant subspace theorem

Template:Use dmy dates Lomonosov's invariant subspace theorem is a mathematical theorem from functional analysis concerning the existence of invariant subspaces of a linear operator on some complex Banach space. The theorem was proved in 1973 by the Russian–American mathematician Victor Lomonosov.

Lomonosov's invariant subspace theorem

Notation and terminology

Let $ℬ(X):=ℬ(X,X)$ be the space of bounded linear operators from some space $X$ to itself. For an operator $T\in ℬ(X)$ we call a closed subspace $M\subset X,M\ne \{0\}$ an invariant subspace if $T(M)\subset M$, i.e. $Tx\in M$ for every $x\in M$.

Theorem

Let $X$ be an infinite dimensional complex Banach space, $T\in ℬ(X)$ be compact and such that $T\ne 0$. Further let $S\in ℬ(X)$ be an operator that commutes with $T$. Then there exist an invariant subspace $M$ of the operator $S$, i.e. $S(M)\subset M$.

References

• Template:Rudin Walter Functional Analysis