# Putnam Exam 2005

#### Putnam Exam 2005

 [A1] Show that every positive integer is a sum of one or more numbers of the form 2^r 3^s, where r and s are nonnegative integers and no summand divides another. (For example, 23 = 9 + 8 + 6.) [A2] Let \mathbf{S} = \{(a,b) | a = 1, 2, \dots,n, b = 1,2,3\}. A \emph{rook tour} of \mathbf{S} is a polygonal path made up of line segments connecting points p_1, p_2, \dots, p_{3n} in sequence such that [(i)] p_i \in \mathbf{S}, [(ii)] p_i and p_{i+1} are a unit distance apart, for 1 \leq i <3n, [(iii)] for each p \in \mathbf{S} there is a unique i such that p_i = p. How many rook tours are there that begin at (1,1) and end at (n,1)? (An example of such a rook tour for n=5 was depicted in the original.) [A3] Let p(z) be a polynomial of degree n, all of whose zeros have absolute value 1 in the complex plane. Put g(z) = p(z)/z^{n/2}. Show that all zeros of g'(z) = 0 have absolute value 1. [A4] Let H be an n \times n matrix all of whose entries are \pm 1 and whose rows are mutually orthogonal. Suppose H has an a \times b submatrix whose entries are all 1. Show that ab \leq n. [A5] Evaluate \int_0^1 \frac{\ln(x+1)}{x^2+1}\,dx. [A6] Let n be given, n \geq 4, and suppose that P_1, P_2, \dots, P_n are n randomly, independently and uniformly, chosen points on a circle. Consider the convex n-gon whose vertices are P_i. What is the probability that at least one of the vertex angles of this polygon is acute? [B1] Find a nonzero polynomial P(x,y) such that P(\lfloor a \rfloor, \lfloor 2a \rfloor) = 0 for all real numbers a. (Note: \lfloor \nu \rfloor is the greatest integer less than or equal to \nu.) [B2] Find all positive integers n, k_1, \dots, k_n such that k_1 + \cdots + k_n = 5n-4 and \frac{1}{k_1} + \cdots + \frac{1}{k_n} = 1. [B3] Find all differentiable functions f: (0, \infty) \to (0, \infty) for which there is a positive real number a such that f' \left( \frac{a}{x} \right) = \frac{x}{f(x)} for all x > 0. [B4] For positive integers m and n, let f(m,n) denote the number of n-tuples (x_1,x_2,\dots,x_n) of integers such that |x_1| + |x_2| + \cdots + |x_n| \leq m. Show that f(m,n) = f(n,m). [B5] Let P(x_1,\dots,x_n) denote a polynomial with real coefficients in the variables x_1, \dots, x_n, and suppose that \left( \frac{\partial^2}{\partial x_1^2} + \cdots + \frac{\partial^2}{\partial x_n^2}\right) P(x_1, \dots,x_n) = 0 \quad \mbox{(identically)} and that x_1^2 + \cdots + x_n^2 \mbox{ divides } P(x_1, \dots, x_n). Show that P=0 identically. [B6] Let S_n denote the set of all permutations of the numbers 1,2,\dots,n. For \pi \in S_n, let \sigma(\pi) = 1 if \pi is an even permutation and \sigma(\pi) = -1 if \pi is an odd permutation. Also, let \nu(\pi) denote the number of fixed points of \pi. Show that \sum_{\pi \in S_n} \frac{\sigma(\pi)}{\nu(\pi) + 1} = (-1)^{n+1} \frac{n}{n+1}. \end{itemize} \end{document}