Number Theory
.7
..
1. Find the last two digits of 77
where the tower contains seven 7’s.
2. Several positive integers are written on a chalk board. One can choose two of them, erase them,
and replace them with their greatest common divisor and least common multiple. Prove that eventually the
numbers on the board do not change.
3.[1966-B2] Prove that among any ten consecutive integers at least one is relatively prime to each of the
others.
4.[1988-B1] A composite (positive integer) is a product ab with a and b not necessarily distinct in
2, 3, 4, .... Show that every composite is expressible as xy + xz + yz + 1. with x, y, and z positive integers.
5.[1989-A1]. How many primes among the positive integers, written in the usual base 10, are such that
their digits are alternating 1’s and 0’s, beginning and ending with 1?
6.[1993-B1]. Find the smallest positive integer n such that for every integer m, with 0 < m < 1993,
there exists an integer k for which
k
m+1
m
< <
.
1993
n
1994
7.[1995-A3] The number d1 d2 . . . d9 has nine (not necessarily distinct) decimal digits. The number
e1 e2 ...e9 is such that each of the nine 9-digit numbers formed by replaying just one of the digits di in
d1 d2 . . . d9 by the corresponding digit ei (1 ≤ i ≤ 9) is divisible by 7. The number f1 f2 . . . f9 is related
to e1 e2 . . . e9 in the same way: that is, each of the nine numbers formed by replaying one of the ei by
the corresponding fi is divisible by 7. Show that, for each i, di − fi is divisible by 7. [For example, if
d1 d2 ...d9 = 199501996 then e6 may be 2 or 9, since 199502996 and 199509996 are multiples of 7].
8.[2000-B2] Prove that the expression
gcd(m, n) n n
m
is an integer for all pairs of integers n ≥ m ≥ 1.
9.[2000-A2] Prove that there exist infinitely many integers n such that n, n + 1, n + 2 are each the sum
of two squares of integers. [Example: 0 = 02 + 02 , 1 = 02 + 12 , and 2 = 12 + 12 .]
10.[1998-A4] Let A1 = 0 and A2 = 1. For n > 2, the number An is defined by concatenating the
decimal expansions of An−1 and An−2 from left to right. For example, A3 = A2 A1 = 10, A4 = A3 A2 = 101,
A5 = A4 A3 = 10110, and so forth. Determine all n such that 11 divides An .
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