Advice in problem solving:

“You must always invert.” — Carl Gustav Jacob Jacobi

“Whenever you can, count.” — Francis Galton

“Each problem that I solved became a rule, which served afterwards to solve other problems.” — Descartes

“By studying the masters, not their pupils.” — Niels Henrik Abel

“Truth is the offspring of silence and meditation. I keep the subject constantly before me and wait ’til the first dawnings open slowly, by little and little, into a full and clear light.” — Isaac Newton