Naturalism is not a major school of thought in the philosophy of mathematics in the 21st century. The three major schools of thought are Platonism, Formalism, and Intuitionism.

According to Platonism, mathematical objects exist independently of the human mind. They are eternal and unchanging, and they can be discovered by the human mind, but they are not created by it.

Formalism is the view that mathematics is a game played with symbols. According to this view, mathematical statements are simply strings of symbols that can be manipulated according to certain rules. The truth or falsity of a mathematical statement is determined by the rules of the game, not by any reference to the real world.

Intuitionism is the view that mathematics is based on self-evident truths. According to this view, mathematical statements are true because they are intuitively obvious. They do not need to be proven or justified by any other means.

The role of mathematics in education is not a major topic in the philosophy of mathematics in the 21st century. The three major topics are the nature of mathematical objects, the foundations of mathematics, and the relationship between mathematics and the natural sciences.

The problem of mathematical beauty is not a major challenge facing the philosophy of mathematics in the 21st century. The three major challenges are the problem of mathematical realism, the problem of mathematical knowledge, and the problem of mathematical progress.

Roger Penrose is not a major philosopher of mathematics in the 21st century. The three major philosophers of mathematics are Penelope Maddy, Timothy Williamson, and Stewart Shapiro.

Gödel's Proof is not a major work in the philosophy of mathematics in the 21st century. It was published in 1931 and is considered to be one of the most important works in the history of mathematics.

Synthese is not a major journal in the philosophy of mathematics. The three major journals are The Journal of Philosophy, Mind, and Philosophical Review.

The World Congress of Philosophy is not a major conference in the philosophy of mathematics. The three major conferences are The Annual Meeting of the Association for Symbolic Logic, The European Conference on Mathematics and Philosophy, and The International Congress of Mathematicians.

The Oxford Centre for the History and Philosophy of Science is not a major research center in the philosophy of mathematics. The three major research centers are The Center for Philosophy of Mathematics at the University of Pittsburgh, The Institute for Logic, Language and Computation at the University of Amsterdam, and The Stanford Center for Logic and the Philosophy of Science.

John Searle is not a major philosopher of mathematics who has died in the 21st century. He is still alive and active. The three major philosophers of mathematics who have died in the 21st century are Saul Kripke, Michael Dummett, and Hilary Putnam.

The problem of mathematical beauty is not a major open problem in the philosophy of mathematics. The three major open problems are the problem of mathematical realism, the problem of mathematical knowledge, and the problem of mathematical progress.

The decline of Platonism is not a major trend in the philosophy of mathematics in the 21st century. Platonism is still a major school of thought, and it is actually experiencing a bit of a revival. The three major trends in the philosophy of mathematics in the 21st century are the rise of naturalism, the increasing interest in mathematical practice, and the growing influence of computer science.

The problem of mathematical beauty is not a major challenge facing the philosophy of mathematics in the 21st century. The three major challenges are the problem of mathematical realism, the problem of mathematical knowledge, and the problem of mathematical progress.