This Math Circle was for middle and high school students who enjoy thinking about mathematical ideas, problems, and puzzles. Those Sunday afternoon sessions stopped in March 2020.

Unfortunately, this Math Circle will not be restarted.

For further information contact: Daniel Shapiro.

Info on floor tilings in the OSU Math Tower: Tessellation Project.

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**2019 – 2020**:

• ~~March 15, 2020: ~~~~Regular Tilings.~~ **CANCELLED**

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• February 16, 2020: Euclid and number theory.

Room: 154 Math Tower

First topic: Euclid’s algorithm yields solutions to equations like 139x – 225y = 1 for integers x, y. This computational method is a major starting point for number theory.

Second topic, led by Margaret Langfield: Triangular numbers, square numbers, and other “figurate” numbers.

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• January 26, 2020: Slide rules.

Room: 154 Math Tower

Before electronic calculators were invented, engineers often used slide rules to find good approximations for products and powers. We will discuss Logarithms and explore how slide rules are constructed using logarithmic ideas.

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• December 8, 2019: Vectors and dots.

Room: 154 Math Tower

What are vectors? How to compute the angle between given vectors v and w ? Or the area of the parallelogram they span?

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• November 10, 2019: Complex numbers (and primes).

Room: 154 Math Tower

Standard integers are viewed as discrete dots on the number line. “Complex integers” are numbers a + bi (where a and b are standard integers), often viewed as dots on the number plane. The prime number 5 is not a complex-prime, because 5 = (2 + i)(2 – i). Which standard primes can be factored this way?

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• October 20, 2019: Prime Numbers.

Room: 154 Math Tower

Prime numbers are the “atoms” for multiplication of numbers, studied since ancient times. We will discuss several of their properties.

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• September 22, 2019: Trigonometry.

Room: 154 Math Tower

Following student requests, we will discuss triangles and standard trig functions. If time allows, we might mention polar coordinates and complex numbers.

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• August 25, 2019: Permutations and symmetry.

Room: 154 Math Tower

Aspects of groups, combinatorics, and geometry.

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**Topics in 2018-2019:**

• April 28, 2019: Cutting & pasting, Moebius, and hexaflexagons.

Room: 154 Math Tower

You might bring a pair of scissors to help with the constructions.

• March 24, 2019: Probability.

Room: 154 Math Tower

In a class of 20 people, how likely is it that two of them have the same birthday?

Flip a coin repeatedly until you get “heads”. On average, how many flips are needed?

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• February 10, 2019: Vectors and matrices.

Room: 154 Math Tower

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• January 27, 2019: Congruence with scissors.

Room: 154 Math Tower

For figures A and B in the plane, when is it possible to cut A into pieces and then rearrange those pieces to make B ? We restrict attention to polygonal figures and straight cuts.

How do those ideas extend: to non-straight cuts, to polyhedra in space, etc.

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• December 2, 2018: Sequences.

Room: 154 Math Tower

We’ll discuss the *difference operator* and polynomial sequences. We might continue with recursive sequences (generalizing Fibonacci), described using the *shift operator*.

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• November 11, 2018: Modular Arithmetic.

Room: 154 Math Tower

Can you find integers x, y satisfying 95x – 58y = 1 ? Euclid’s algorithm provides a systematic way to solve such equations. Given integer k, we will see how those methods provide a tool for solving “congruences” in the system of integers modulo k.

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• October 14, 2018: Complex Numbers.

Room: 154 Math Tower (or to a larger room: Cockins 240)

Elementary school students learn about rational numbers (fractions). The larger system of “real” numbers includes many irrationals, like sqrt{2} and pi. Since real numbers correspond to points on the number line, the reals are useful for geometry. “Complex” numbers are built from the reals together with the “imaginary” number i = sqrt{-1}. Complex numbers correspond to points in the plane, leading to interesting connections with plane geometry.

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• September 16, 2018: Sizes of infinity.

Room: 154 Math Tower

Given two sets, how can we tell whether they have the same size?

(We discuss this topic every year, but there are different variations each time.)

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• August 26, 2018: Stern’s Triangle.

Room: 154 Math Tower

Any fraction a/b has a left-child L(a/b) = a/(a+b) and a right-child R(a/b) = (a+b)/b.

Starting from 1/1, apply L and R to obtain 1/2 and 2/1. The operations L and R applied to those yield four new fractions. Applying L and R repeatedly, which fractions x/y appear in this list?

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