I am happy to be near the midpoint of my Little 2021 Mathematics A-to-Z. It feels like forever since I planned to start this, but it has been a long and a hard year. I am in need of topics for the third quarter of letters, the end of the world ‘Mathematics’, and so I appeal to my kind readers for help.

What are mathematical topics which start with the letters I, C, or S, that you’d like to see me try explaining? Leave a comment, and let me know. I’ll pick the one I think I can be most interesting about. As you nominate things, please also include a mention of your own blog or YouTube channel or book. Whatever other projects you do that people might enjoy. The projects don’t need to be mathematical. The topics don’t need to be either, although I like being able to see mathematics from them.

Here are the topics I’ve covered in past years. I’m willing to consider redoing one of these, if I can find a fresh approach. So don’t be afraid to ask if you think I might do a better job about, oh, cohomology or something.

## I.

**Into**(2015)**Isomorphism**(Leap Day 2016)**Image**(End 2016)**Integration**(2017)**Infinite Monkey Theorem**(2018)**Infimum**(2019)**Imaginary Numbers**(2020)

## C.

**Characteristic**(2015)**Conjecture**(Leap Day 2016)**Cantor’s Middle Third**(End 2016)**Cohomology**(2017)**Commutative**(2018)**Category Theory**(2019)**Complex Numbers**(2020)

## S.

**Step**(2015)**Surjective Map**(Leap Day 2016)**Smooth**(End 2016)**Sárközy’s Theorem**(2017)**Sorites Paradox**(2018)**Sample Space**(2019)**Statistics**(2020)

(Please note: there’s nothing I can do with cohomology. I did my best and that’s how it came out.)

All the Little 2021 A-to-Z essays should be at this link. And if you like, all of my A-to-Z essays, for every year, should be at this link. Thanks for reading, and thanks for suggesting things.

A few ideas, maybe…

Icosahedron or Intersection

Convexity or Continuity / Continuous Function

Stokes’ Theorem or Square Root

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Great suggestions! Thank you.

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I inverses

C counting

S subtraction

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Great suggestions, thank you! I’m particularly taken with subtraction, since I do seem to be working on all the basic arithmetic functions this time around … have to see if I can think of an interesting perspective, though.

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I: Ideal

C: Contour

S: Symmetry

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Thank you! Great topics particularly as I know I’ve mentioned contours in a few places without more than handwaving at what they are.

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