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# Bijections

Bijections, surjections, and injections are three types of functions which associate the elements between two sets. For example, each word in this sentence can be mapped to exactly one in the last.

\[\large\sum _{ i=1 }^{ n }{ \left\lfloor \frac { A }{ B } i \right\rfloor } +\sum _{ j=1 }^{ m }{ \left\lfloor \frac { B }{ A } j \right\rfloor } = f(n)f(m)\]

Let \(A,B\) be odd natural numbers that are relatively prime, and let \(n= \frac { B-1 }{ 2 } \), \(m=\frac { A-1 }{ 2 } \) such that the equation above is fulfilled. If \(f\) is a linear function with \( f(1) > 0 \), what is the sum of the coefficients of \(f(x)\)?

All 20 diagonals are drawn in a regular octagon. At how many distinct points in the interior of the octagon (not on the boundary) do two or more diagonals intersect?

**Details and assumptions**

A regular octagon has \( { 8 \choose 2} - 8 = 28 - 8 = 20\) diagonals.

How many permutations \(\sigma\) of the set \( \{1, 2, \ldots, 15\} \) are there such that \( \sigma (1) = 1, \lvert \sigma (n) - \sigma (n-1) \rvert \leq 2 \) for \( 2 \leq n \leq 15 \)?

**Details and assumptions**

\(\sigma(n)\) denotes the \(n^{th}\) position of the permutation.

Brilli the ant is considering the complexities of a 5-dimensional universe. She wants to count the number of integer points that are distance \( \sqrt{n} \) away from the origin. Let \(T_n\) be the set of ordered 5-tuples of integers \((a_1,a_2,a_3,a_4,a_5)\) such that \(a_1^2 + a_2^2 + a_3^2 + a_4^2 + a_5^2 = n.\) However, being an ant, she doesn't have enough toes to record numbers that are above 10. Let \(D_n\) be the units digit of \(\vert T_n \vert.\)

Determine \(\sum_{i = 1}^{5678} D_i.\)

**Details and assumptions**

Note: You are not asked to find the last digit of the sum, but the sum of all the last digits.

For a set \(S\), \( | S | \) denotes the number of elements in \(S\). You can read up on set notation.

What is the minimum number of slices the cake cutter needs to slice the cake into such that either \(28\) or \(29\) people can attend the party and the cake can still be distributed evenly?

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