Ramsey's Theorem and the Pigeonhole Principle in Intuitionistic Mathematics
Wim Veldman, Bezem · Journal of the London Mathematical Society · 1993
At first sight, the argument which F. P. Ramsey gave for (the infinite case of) his famous theorem from 1927, is hopelessly unconstructive. If suitably reformulated, the theorem is true intuitionistically as well as classically: we offer a proof which should convince both the classical and the intuitionistic reader.