David Radcliffe

Mathematician, software developer, engineer, SJW. Co-founder of GogyUp (gogyup.com). Formerly on Twitter as @daveinstpaul.

Blog: https://radcliffe.dev/index.php/contact/
Github: https://github.com/Radcliffe

Last Notes

2025-12-05 15:13:39 UTC

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Here is my writeup. I am preparing an article on lower bounds of digital sums.
https://media.mathstodon.xyz/media_attachments/files/115/667/600/913/213/502/original/d42b96d705fe4f3e.png

2025-12-04 19:04:30 UTC

Everybody knows that the sum of the decimal digits of a multiple of 9 is also a multiple of 9. But did you know that the digit sum of a positive multiple of 999 is always 27 or more?

2025-07-15 02:49:21 UTC

I made a puzzle app to explore an unsolved problem in group theory. I tried to make it intelligible to non-mathematicians. What do you think? https://radcliffe.github.io/01matrixpuzzle/

2025-07-07 21:55:36 UTC

I have a question that's too silly for Math Overflow. There's a famous trick for swapping two integers in three steps, without using a temporary variable:
a ← a + b
b ← a − b
a ← a − b
This can be generalized to cyclic permutations, e.g.
a ← a + b + c
b ← a − b − c
c ← a − b − c
a ← a − b − c
How many steps are needed to realize an arbitrary permutation? Assume that each step replaces one variable with a fixed integer combination of all of the variables.

2025-01-29 17:44:40 UTC

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I thought that she was talking about academic feminism in particular. The aphorism seems absurd if it's taken to mean all tools used by an oppressor.

2024-09-19 20:53:23 UTC

Find a sequence of 1000 positive integers in arithmetic progression whose digit sums in base 10 are equal.

2024-09-09 13:56:11 UTC

Who is a great math video creator that few people know about? I'm a fan of 3blue1brown and Mathologer, but they have enough supporters already.