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Who you putting on a math Mount Rushmore?

New post: Kernel and Cokernel johndcook.com/blog/2025/04/1…

“The simplest solution is to enlarge our universe as needed.” Line from a math book that sounds delusional out of context.

If we push the Gregorian calendar back to ancient dates, how much of a difference does it make? johndcook.com/blog/2025/04/2…

'Generalisations of a structure can look very different depending on which particular description of it we started with.' -- Eugenia Cheng

Encryption in groups of unknown order johndcook.com/blog/2021/08/2…

Let R be an integral domain. Then R is a field iff every finitely generated R-module is free.

A regular n-gon is constructible by straightedge and compass iff φ(n) is a power of 2. φ = Euler's totient function.

For elliptic curves, it's easier to prove that addition is commutative than to prove it is associative.

A category is Cartesian closed if it has all finite products and exponentials.

Morse code and the limits of perception news.ycombinator.com/item?id=437802…

New post: The Equipentatonic Scale A little music theory and a little number theory johndcook.com/blog/2025/04/2…

Quaternion square roots johndcook.com/blog/2021/01/0…

Every group with fewer than 60 elements is solvable.

The direct product of groups A and B is isomorphic to the direct product in the opposite order. Not necessarily true for semidirect products.

If a group G acts on a set A, the stabilizer of an element a of A is the subgroup of G whose action leaves a fixed.

New post: Transpose and Adjoint johndcook.com/blog/2025/04/2…

New post: Adjunctions johndcook.com/blog/2025/04/2…