# Mathematical Proof

I assumed the commutative law of multiplication when I proved the Big

Distributive Law.

This allowed me to use the left distributive law alone.

(When you have commutativity, you only really need one distributive law.)

Don’t assume commutativity. Redo the proof so that it works with noncommutataive algebra, such as matrix algebra.

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Describe a strategy for computing the left side in a computer language that won’t allow you to throw nested loops on the fly.

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The proof the Big Distributive Law completes the proof of the

Fundamental

Theorem of Arithmetic.

The FTA gives us many irrationality proofs.

How about sqrt{3} – sqrt{2}.

T = sqrt{3} – sqrt{2}

T^2 = 3 – 2sqrt{6} + 2

(5 – T^2) / 2 = sqrt{6}

This if T were rational, sqrt{6} would be rational.

But the proof for the sqrt{2} works for sqrt{6}.

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(sqrt{3} – sqrt{2})(sqrt{3} + sqrt{2}) = 1

Thus, if either of the quantities sqrt{3} – sqrt{2} or sqrt{3} +

sqrt{2}

is rational, so is the other. Then, so is their sum and difference.

So that would make both sqrt{2} and sqrt{3} rational, contradiction

either way.

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Multiply out the polynomial

(x + sqrt{3} + sqrt{2})*(x + sqrt{3} – sqrt{2})

*(x – sqrt{3} + sqrt{2})*(x – sqrt{3} – sqrt{2})

Conclude that this is a polynomial with integer coefficients and

leading coefficient

equal to 1.

A polynomial with integer coefficients and leading coefficient

equal to 1 is called a **monic polynomial**.

Claim: The roots of a monic polynomial are either integers or

irrational.

This applies to sqrt{2} (root of x^2 – 2)

to cube root of 2 (root of x^3 – 2)

and by the exercise, to sqrt{3} – sqrt{2}

…and to much else besides.

Fix a monic polynomial p(x) = x^n + sum_{i=0}^{n-1} a_i x^i.

Without loss of generality, assume that a_0 not= 0.

(If not, divide out some x’s.)

Now assume for contradiction that s/t is a root of p: p(s/t)=0 and |

t|not=1.

(Assume s/t is in lowest terms.)

Consider the expression t^n * p(s/t). The value of this expression

equals 0,

by the assumption. Let Q be a prime that divides t.

t^n * p(s/t) = t^n * ((s/t)^n + sum_{i=0}^{n-1} a_i (s/t)^i)

= s^n + sum_{i=0}^{n-1} a_i s^i*t^{n-i}

Every one of these summands is divisible by Q except the first.

…flesh out the details and finish the proof.

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You learned in HS that rational numbers have repeating decimals and

vice versa.

1/7 = .

142857142857142857142857142857142857142857142857142857142857142857…

. 1 4

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7 ) 1 . 0 0 0 0 0 0

7

—–

3 0

2 8

—

2

:

Repeating decimals for rational numbers is a special case of a fact about computations you can do with finite state machines

(with no input)

The denominator gives me an upper bound for when the repeating will start.

It also gives me a bound on the length of the cycle.

HOMEWORK: Divise a notation so you can turn this into a real proof.

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