(1) Linear algebra. E.g., part of it is part of the crucial, core math of my startup. Also as we see today, Jack J Dongarra just won a Turing Award, and one o

(1) Linear algebra. E.g., part of it is part of the crucial, core math of my startup. Also as we see today, Jack J Dongarra just won a Turing Award, and one of his best accomplishments is what he did for LINPACK, numerical linear algebra. (2) Integer linear programming set covering. A super nice way to get likely very close to optimal solutions to some really nasty real world problems in combinatorial optimization. (3) My favorite text editor, Kedit. It is a PC version of the IBM text editor XEDIT, written by an IBM guy in France, apparently on his own time. It used Rexx as a macro language, and Kedit uses Kexx, its version of Rexx, as its macro language. XEDIT and Kedit are elegantly designed; the macro languages are also elegant. Kedit is my most important tool. (3) Hierarchical file systems, e.g., Microsoft's NTFS (new technology file system). I just find the hierarchy in the file system as a natural a taxonomic hierarchy for organizing information. (4) Yes, once I used Monte Carlo simulation of a continuous time, discrete state space Markov process to estimate the surviveability of the US SSBN fleet under a special scenario of global nuclear war limited to sea. So, it was potentially an enormously complicated calculation but due to the simplification of the Markov assumption, how the continuous time was handled with a Poisson process, and the Monte Carlo to do the numerical part, I got the whole project done in just two weeks.