The Feynman-Hellman theorem is concerned with a Hamiltonian which depends on some parameter X. It states that:

Richard Feynman’s Technique for Learning Something New: An Animated Introduction

Algebra for the Practical Man (THE PLEASURE OF FINDING THINGS OUT with RICHARD FEYNMAN) My cousin, at that time, who was three years older, was in high school and was having considerable difficulty with his algebra and had a tutor come, and I was allowed to sit in a corner while the tutor would try to teach my cousin algebra, problems like 2x plus something. I said to my cousin then, “What’re you trying to do?” You know, I hear him talking about x. He says, ”What do you know-2x 7 is equal to 15,

ファインマン先生(Richard phillips Feynman) 1-4