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 Aug 11 awarded Yearling May 22 awarded Commentator May 22 comment A series involving harmonic numbers Some of these symbolic algorithms have "certification methods" attached to them -- e.g., the Wilf-Zeilberger method. There are some add-on packages for Mathematica that show such certificates. This is probably not what you mean exactly by "derivation method". But the fact is that it does become a fully automated proof. Some computer algorithms allow such certifications, others do not. Once should always check numerically that the symbolic result returned agrees with the numeric approximation, of course. May 22 answered A series involving harmonic numbers May 21 awarded Critic May 20 revised Free groups of rotations of the sphere added 49 characters in body May 20 asked Free groups of rotations of the sphere May 19 answered Looking for explanation of Banach-Tarski Proof, preferably by visual methods “Video, Pictures, Diagrams…” May 19 comment free groups of rotations So that assertion is too strong. What about this: If A and B are noncommuting rotations of infinite order, then G, the group they generate, has a free subgroup of rank 2. May 19 accepted free groups of rotations May 19 comment free groups of rotations Many thanks. It might be nice to have a specific examples of the 3x3 matrices that do not commute but satisfy a word such as you suggested, but I don't really need that. It is enough to know that the theorem is false, and so this adds to the value of the many theorems that say that certain pairs of rotations are independent. May 18 comment free groups of rotations 30 years on, I am working (with G. Tomkowicz) on revising the BTP book. Big job. Almost finished. That is where this question arose. May 18 asked free groups of rotations May 6 answered Solving quadratic or higher degree congruence with very large modulus. Mar 14 revised Prove that every outer planar graph is $3$-choosable added 408 characters in body Mar 14 answered Prove that every outer planar graph is $3$-choosable Mar 12 answered Examples of non-hamiltonian decomposable graphs Mar 8 revised Worst case in decanting puzzles (pouring water from one jug to others). I forgot that I had made the assumption that A0+B0+C0 is at most A, where A0, etc is the initial state. Mar 7 awarded Curious Mar 6 asked Worst case in decanting puzzles (pouring water from one jug to others).