Fredholm 1903: for compact operator K on Banach space, either (I-K)x = y has unique solution for all y, or homogeneous (I-K)x = 0 has nontrivial solutions; basis of integral-equation theory.
Fredholm 1903: for compact operator K on Banach space, either (I-K)x = y has unique solution for all y, or homogeneous (I-K)x = 0 has nontrivial solutions; basis of integral-equation theory.