Highlights from the letter:
In the period 1945-1980, it was generally believed that patent law did not pertain to software. However, it now appears that some people have received patents for algorithms of practical importance--e.g., Lempel-Ziv compression and RSA public key encryption--and are now legally preventing other programmers from using these algorithms.It is only a 2-page letter, and I encourage you to read it.
This is a serious change from the previous policy under which the computer revolution became possible, and I fear this change will be harmful for society.
Congress wisely decided long ago that mathematical things cannot be patented. Surely nobody could apply mathematics if it were necessary to pay a license fee whenever the theorem of Pythagoras is employed.