Mathematical Problem Solving by Alan H. Schoenfeld.
In his chapter on thinking mathematically (1992), Alan Schoenfeld suggests that whilst the idea of problems has been a part of the maths curriculum for ever, problem solving has not. And furthermore there are different definitions of what a problem is, and hence what problem solving means.

Perhaps the best way to summarize my problem-solving work (see, e.g., Schoenfeld, 1985, 1992) is that it consisted of a decade-long series of design experiments aimed at understanding and enhancing students’ mathematical problem solving.

Amazon.com: Mathematical Thinking and Problem Solving (Studies in Mathematical Thinking and Learning Series) (9780805809909): Schoenfeld, Alan H., Sloane, Alan H.: Books.

Education and career. Schoenfeld was raised in New York City, studying at Queen's College (BA 1968) before moving to Stanford for research in pure mathematics (MS 1969, Ph.D 1973 on topology and measure theory). During his graduate studies he became increasingly interested in the teaching and learning of mathematics, particularly of non-routine problem solving.

Reflections on Problem Solving Theory and Practice. Alan H. Schoenfeld. 1. University of California, Berkeley, CA, USA. Abstract: In this article, the author reflects on the current state of mathematical problem solving, both in theory and in instruction. The impact of the book Mathematical Problem.

DOCUMENT RESUME. ED 229 248 SE 041 449 AUTHOR Schoenfeld, Alan H. TITLE Problem Solving in the Mathematics Curriculum. A. Report, Recommendations, and an Annotated Bibliography. MAA Notes, Number 1. INSTITUTION. Mathematical Association of America, Washington.

Mathematics educators using problem solving for evaluation have an issue phrased by Alan H. Schoenfeld: How can one compare test scores from year to year, when very different problems are used? (If similar problems are used year after year, teachers and students will learn what they are, students will practice them: problems become exercises, and the test no longer assesses problem solving).