Olympiad problems

2017 Putnam, A2

Let $Q_0(x)=1$, $Q_1(x)=x,$ and \[Q_n(x)=\frac{(Q_{n-1}(x))^2-1}{Q_{n-2}(x)}\] for all $n\ge 2.$ Show that, whenever $n$ is a positive integer, $Q_n(x)$ is equal to a polynomial with integer coefficients.

1985 Putnam, A6

Tags: Putnam , algebra , polynomial

If $p(x)=a_{0}+a_{1} x+\cdots+a_{m} x^{m}$ is a polynomial with real coefficients $a_{i},$ then set $$ \Gamma(p(x))=a_{0}^{2}+a_{1}^{2}+\cdots+a_{m}^{2}. $$ Let $F(x)=3 x^{2}+7 x+2 .$ Find, with proof, a polynomial $g(x)$ with real coefficients such that (i) $g(0)=1,$ and (ii) $\Gamma\left(f(x)^{n}\right)=\Gamma\left(g(x)^{n}\right)$ for every integer $n \geq 1.$

1951 Putnam, A2

Tags: Putnam

In the plane, what is the locus of points of the sum of the squares of whose distances from $n$ fixed points is a constant? What restrictions, stated in geometric terms, must be put on the constant so that the locus is non-null?

2021 Putnam, B6

Tags: Putnam , Putnam 2021

Given an ordered list of $3N$ real numbers, we can trim it to form a list of $N$ numbers as follows: We divide the list into $N$ groups of $3$ consecutive numbers, and within each group, discard the highest and lowest numbers, keeping only the median. \\ Consider generating a random number $X$ by the following procedure: Start with a list of $3^{2021}$ numbers, drawn independently and unfiformly at random between $0$ and $1$. Then trim this list as defined above, leaving a list of $3^{2020}$ numbers. Then trim again repeatedly until just one number remains; let $X$ be this number. Let $\mu$ be the expected value of $\left|X-\frac{1}{2} \right|$. Show that \[ \mu \ge \frac{1}{4}\left(\frac{2}{3} \right)^{2021}. \]

1951 Putnam, A7

Tags: Putnam

Show that if the series $a_1 + a_2 + a_3 + \cdots + a_n + \cdots$ converges, then the series $a_1 + a_2 / 2 + a_3 / 3 + \cdots + a_n / n + \cdots$ converges also.

1981 Putnam, A2

Tags: Putnam , combinatorics , chess

Two distinct squares of the $8\times8$ chessboard $C$ are said to be adjacent if they have a vertex or side in common. Also, $g$ is called a $C$-gap if for every numbering of the squares of $C$ with all the integers $1, 2, \ldots, 64$ there exist twoadjacent squares whose numbers differ by at least $g$. Determine the largest $C$-gap $g$.

1990 Putnam, B5

Tags: algebra , polynomial , Putnam , college contests

Is there an infinite sequence $ a_0, a_1, a_2, \cdots $ of nonzero real numbers such that for $ n = 1, 2, 3, \cdots $ the polynomial \[ p_n(x) = a_0 + a_1 x + a_2 x^2 + \cdots + a_n x^n \] has exactly $n$ distinct real roots?

1942 Putnam, B3

Tags: Putnam , partial differential equations , differential equation

Given $x=\phi(u,v)$ and $y=\psi(u,v)$, where $ \phi$ and $\psi$ are solutions of the partial differential equation $$(1) \;\,\;\, \; \frac{ \partial \phi}{\partial u} \frac{\partial \psi}{ \partial v} - \frac{ \partial \phi}{\partial v} \frac{\partial \psi}{ \partial u}=1.$$ By assuming that $x$ and $y$ are the independent variables, show that $(1)$ may be transformed to $$(2) \;\,\;\, \; \frac{ \partial y}{ \partial v} =\frac{ \partial u}{\partial x}.$$ Integrate $(2)$ and show how this effects in general the solution of $(1)$. What other solutions does $(1)$ possess?

1985 Putnam, A4

Tags: Putnam

Define a sequence $\left\{a_{i}\right\}$ by $a_{1}=3$ and $a_{i+1}=3^{a_{i}}$ for $i \geq 1.$ Which integers between $00$ and $99$ inclusive occur as the last two digits in the decimal expansion of infinitely many $a_{i} ?$

2016 Putnam, A1

Tags: Putnam , Putnam 2016 , Putnam easy , Putnam number theory

Find the smallest positive integer $j$ such that for every polynomial $p(x)$ with integer coefficients and for every integer $k,$ the integer \[p^{(j)}(k)=\left. \frac{d^j}{dx^j}p(x) \right|_{x=k}\] (the $j$-th derivative of $p(x)$ at $k$) is divisible by $2016.$

1958 November Putnam, B6

Tags: Putnam , graph , paths

Let a complete oriented graph on $n$ points be given. Show that the vertices can be enumerated as $v_1 , v_2 ,\ldots, v_n$ such that $v_1 \rightarrow v_2 \rightarrow \cdots \rightarrow v_n.$

1949 Putnam, B3

Tags: Putnam , curves

Let $K$ be a closed plane curve such that the distance between any two points of $K$ is always less than $1.$ Show that $K$ lies in a circle of radius $\frac{1}{\sqrt{3}}.$

1972 Putnam, B1

Tags: Putnam , Sequences

Let $\sum_{n=0}^{\infty} \frac{x^n (x-1)^{2n}}{n!}=\sum_{n=0}^{\infty} a_{n}x^{n}$. Show that no three consecutive $a_n$ can be equal to $0$.

2012 Putnam, 3

Tags: Putnam , function , limit , algebra , functional equation , topology , college contests

Let $f:[-1,1]\to\mathbb{R}$ be a continuous function such that (i) $f(x)=\frac{2-x^2}{2}f\left(\frac{x^2}{2-x^2}\right)$ for every $x$ in $[-1,1],$ (ii) $ f(0)=1,$ and (iii) $\lim_{x\to 1^-}\frac{f(x)}{\sqrt{1-x}}$ exists and is finite. Prove that $f$ is unique, and express $f(x)$ in closed form.

1973 Putnam, B1

Tags: Putnam , Integers

Let $a_1, a_2, \ldots a_{2n+1}$ be a set of integers such that, if any one of them is removed, the remaining ones can be divided into two sets of $n$ integers with equal sums. Prove $a_{1}=a_2 =\cdots=a_{2n+1}.$

2021 Putnam, A5

Tags: Putnam , Putnam 2021

Let $A$ be the set of all integers $n$ such that $1 \le n \le 2021$ and $\text{gcd}(n,2021)=1$. For every nonnegative integer $j$, let \[ S(j)=\sum_{n \in A}n^j. \] Determine all values of $j$ such that $S(j)$ is a multiple of $2021$.

2022 Putnam, B2

Tags: Putnam , Putnam 2022 , linear algebra , combinatorial geometry

Let $\times$ represent the cross product in $\mathbb{R}^3.$ For what positive integers $n$ does there exist a set $S \subset \mathbb{R}^3$ with exactly $n$ elements such that $$S=\{v \times w: v, w \in S\}?$$

1989 Putnam, A2

Tags: Putnam , integration , college contests

Evaluate $\int^{a}_{0}{\int^{b}_{0}{e^{max(b^{2}x^{2},a^{2}y^{2})}dy dx}}$

1968 Putnam, A1

Tags: integration , algebra , polynomial , Putnam , calculus , college contests

Prove $ \ \ \ \frac{22}{7}\minus{}\pi \equal{}\int_0^1 \frac{x^4(1\minus{}x)^4}{1\plus{}x^2}\ dx$.

1986 Putnam, A1

Tags: Putnam

Find, with explanation, the maximum value of $f(x)=x^3-3x$ on the set of all real numbers $x$ satisfying $x^4+36\leq 13x^2$.

2014 Contests, 1

Tags: Putnam , function , relatively prime , prime factorization , Putnam calculus , Putnam 2014

Prove that every nonzero coefficient of the Taylor series of $(1-x+x^2)e^x$ about $x=0$ is a rational number whose numerator (in lowest terms) is either $1$ or a prime number.

2005 Putnam, B3

Tags: Putnam , function , integration , logarithms , algebra , functional equation , absolute value

Find all differentiable functions $f: (0,\infty)\mapsto (0,\infty)$ for which there is a positive real number $a$ such that \[ f'\left(\frac ax\right)=\frac x{f(x)} \] for all $x>0.$

1940 Putnam, A5

Tags: Putnam , conics , ellipse

Prove that the simultaneous equations $$x^4 -x^2 =y^4 -y^2 =z^4 -z^2$$ are satisfied by the points of $4$ straight lines and $6$ ellipses, and by no other points.

2006 Putnam, B3

Tags: Putnam , rotation , induction , floor function , ceiling function , college contests

Let $S$ be a finite set of points in the plane. A linear partition of $S$ is an unordered pair $\{A,B\}$ of subsets of $S$ such that $A\cup B=S,\ A\cap B=\emptyset,$ and $A$ and $B$ lie on opposite sides of some straight line disjoint from $S$ ($A$ or $B$ may be empty). Let $L_{S}$ be the number of linear partitions of $S.$ For each positive integer $n,$ find the maximum of $L_{S}$ over all sets $S$ of $n$ points.

2006 Putnam, B6

Tags: Putnam , limit , factorial , Harvard , college , inequalities , integration

Let $k$ be an integer greater than $1.$ Suppose $a_{0}>0$ and define \[a_{n+1}=a_{n}+\frac1{\sqrt[k]{a_{n}}}\] for $n\ge 0.$ Evaluate \[\lim_{n\to\infty}\frac{a_{n}^{k+1}}{n^{k}}.\]