See also the
Dr. Math FAQ:
0.9999 = 1
0 to 0 power
n to 0 power
0! = 1
dividing by 0
Browse High School Number Theory
Stars indicate particularly interesting answers or
good places to begin browsing.
Selected answers to common questions:
Infinite number of primes?
Testing for primality.
What is 'mod'?
- Sums Divisible by 11 [10/10/2001]
Why is the sum of a number with an even number of digits and that same
number written in reverse always divisible by 11?
- Sums of Consecutive Integers [01/03/2001]
How many different ways can 2000 be expressed as the sum of two or more
consecutive positive integers?
- Sums of Consecutive Integers [02/04/2001]
What numbers can be expressed as the sum of a string of consecutive
- Sums of Consecutive Integers with Digital Sums [01/27/2004]
Find all sets of positive consecutive integers that sum to 100, and
whose digits sum to greater than 30.
- Sums of Consecutive Numbers [06/20/2002]
In what way(s) can 1000 be expressed as the sum of consecutive
- Sums of Consecutive Odd vs. Even Integers [04/15/2002]
Can the sum of two consecutive even integers ever equal the sum of
two consecutive odd integers? Why or why not?
- Sums of Consecutive Positive Integers [03/02/2001]
Why are the powers of 2 the only numbers you cannot get as the sum of
a series of consecutive positive integers?
- Sums of Sets of Prime Numbers [01/07/2003]
Given several sets of prime numbers, use each of the nine non-zero
digits exactly once. What is the smallest possible sum such a set
- Sums of Square Integers Puzzle [07/01/2002]
How many numbers from 1-100 can be expressed as the sum of the squares
of two positive integers?
- Sums of Three Squares [05/18/1998]
What numbers cannot be expressed as the sum of three squares?
- Sum Square Root Search [03/06/2015]
An adult wonders for what integer is the sum of the digits of its square equal to the
square root of the number. With some programming premised on logarithmic thinking,
Doctor Ali provides numerical solutions.
- Synthetic Division [11/13/1997]
Why does synthetic division work?
- System-Level Programming and Base 2 [05/03/2001]
In computer programming, I have a result that contains several values,
always a power of 2 (2^2, 2^3, 2^4). If my value is 2^3, 2^4, 2^6 304,
how can I tell if 2^3 exists in 304?
- Systems with More Variables than Equations [12/11/2002]
How do I solve a system that has three variables and only two equations, such as 187y + 98x + 45z = 48 and 2y + 9x + 3z = 198?
- Taylor Expansion [11/21/2001]
Can you give me the proof of this statement: arcsin(x) = x + 1/2 (x^3/ 3)
+ (1/2)(3/4)(x^5/5) + (1/2)(3/4)(5/6)(x^7/7) + ...? The basis of the
calculation is a Taylor series.
- A Theorem to Find Lattice Points [6/1/1996]
What are the conditions under which the line ax+by=c will contain lattice
- Three Number Theory Questions [10/25/1999]
Find the sum of the digits in 4444^4444; find how many times the digit 1
occurs from 1 up to 10,000,000,000; find 3 integers greater than 5^100
that are factors of (5^1985)-1.
- TI-86 Base Conversion Program [03/19/2002]
I have finished writing a program that can convert any number in any base
(one-ten) to base ten. Now I am writing a program to convert any number
in base ten to a given base.
- Towers of Hanoi [10/08/2000]
Can you prove the formula 2^n - 1 for the least number of moves it takes
to move all n discs to another peg in Towers of Hanoi?
- Trailing Zeros and Zero Factorial [04/07/1998]
How many trailing zeros are there for 100! ?
- Transfinite Arithmetic [10/28/1997]
What is transfinite arithmetic? I pretty much know what it means, but I
am having trouble applying it to aleph-null.
- Transfinite Numbers [11/07/1997]
I know that Georg Cantor discovered transfinite numbers, but what are
- Triangle Perimeters [12/15/1998]
How many triangles with integer sides have a given perimeter? How does
the triangle inequality enter into the proof?
- Triangle Proofs [06/28/1998]
The sides of a triangle are a,b,c; prove that (a+b+c)^3 >= 27(b+c-
- Triangular Numbers [07/07/1998]
How do you know a number is triangular? How is n/2(n+1) derived?
- Triangular Numbers [08/23/2003]
How do I show that the sum of any two consecutive triangular numbers
is always a square number?
- Triangular Numbers That are Perfect Squares [09/07/99]
How can I find and prove a general formula which will give me numbers
which are both triangular numbers and perfect squares?
- Triangular Triples: Means that Are Not So Average [04/28/2012]
A student finds several pairs of triangular numbers that average to a third one, and so
wonders how many more such triples exist — and how to generate
them. With a few inspired variable substitutions and some modular arithmetic, Doctor
Jacques responds, then suggests a few new questions to explore.
- Tribonacci Numbers [11/11/2000]
Is there an implicit formula to calculate the nth Tribonacci number?
Also, is there a formula to find the sum of the first n Tribonacci
- Trick for Numbers Divisible by 3 or 9 [02/24/1998]
Proof of a trick for numbers that are divisible by 3 or 9.
- Twin Prime Numbers [3/11/1996]
You know that a prime number is a whole number greater than 1 whose only
whole number divisors are 1 and itself. You may not know that there are
also such things as twin prime numbers....
- Twin Primes [12/24/1997]
Are there any studies being conducted on twin primes?
- Two Prime Numbers with 400-Digit Product [07/29/2006]
Find two prime numbers whose product is a 400-digit number.
- Two's Complement [07/13/1999]
What is two's complement and how is it used?
- Uncountable Infinitude, Illogically Concluded [11/21/2010]
If a rational number can be found between any two irrationals, and the set of
irrationals are uncountably infinite, does that mean that the rationals are also
uncountable? Doctor Peterson points up the flaw in a student's assumption about what
to conclude from a failed mapping.
- Uncountable Numbers [10/20/1997]
Why are the real numbers between 0 and 1 uncountable?
- Undefined Fractions [02/19/2002]
Why is a fraction with a denominator of zero called "undefined"?
- Unique Decomposition of Pythagorean Primes [05/19/2002]
Is it true that a Pythagorean prime (i.e., a natural prime that can
be expressed as a sum of squares of two integers) can be expressed
as a sum of two squares in one and only one way?
- Unknown Numbers and a Venn Diagram [11/26/2001]
The GCF of two numbers is 20 and the LCM is 840. One of the numbers is
120. Explain how to find the other number and use the Venn diagram method
- Unsolvable and Unsolved Problems [02/19/1998]
What's the difference between problems like Squaring the Circle and
Goldbach's Conjecture or the Collatz Problem?