My first job out of grad schools was with a small company called Analysis & Technology. They were located in Mystic, CT, later North Stonington. Their business was doing studies and writing manuals for the Navy. Our biggest customer the Sub Base in Groton. I worked mostly on problems of tactics and applying statistics. Some others worked on sonar, and a book that was sort of a sonar primer showed how the wave equation is developed, and someone had a few pages from a math book that included a normal mode solution for the wave equation. I've been thinking lately that I should have taken the opportunity to look about those things and maybe learn a bit more about differential equations (one of the nearly blank spots in my math resume).
So, the other day, I Googled.
This was published about three years after I left A&T for PepsiCo. Note that it says OK for public release down at the bottom.
I downloaded this and read the first five pages or so. I really don't have the background in differential equations to follow the derivations. I got to wondering if the text for the one semester of DE that I did have would contain anything useful. It's up in the attic. So, I went up into the attic. I know about about where the boxes of textbooks are, but there are a bunch of other boxes in the way.
Some of them contain journals from back in the day. When I put them up there, journals had resale value since libraries had to replace lost volumes, and now and then, a new library is created. Today, however, basically any serious journal can be read from anywhere via the internet. So a hundred pounds of journals are carried down, and will eventual be recycled. And, since they were also in the way, the same for the reserve, I.e. worn out, set of sails for the Capri 22 which I sold about 15 years ago.
My textbook is still in the attic, and I'm too tired to do math. Maybe I'll look at one of the Rex Stout paperbacks that I also stumbled across.