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'?
- 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?
- 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?
- Uses of Bases Other Than Base 10 [09/28/2004]
Why are there different math bases and what would they be used for?
- Uses of Imaginary Numbers [03/24/1997]
Can you tell me a real-life application of imaginary numbers?
- Using a Calculator in Other Bases [05/16/2000]
Can a calculator be used to add non-decimal numbers? For example, 27
octal + 65 octal.
- Using Algebra to Find Lucky Numbers [12/26/2007]
A lucky number is a positive integer which is 19 times the sum of its
digits. How many different lucky numbers are there?
- Using Binomial Expansion to Evaluate [2 + sqrt(3)]^50 [11/29/2006]
I've used a computer to evaluate [2 + sqrt(3)]^50 and the answer is
extremely close to being an integer. I've tried various expansions of
the expression to try and determine why it's so close to an integer,
but haven't gotten anywhere. Do you have any idea why?
- Using Mod to Find Digits in Large Numbers [12/10/1996]
Find the last two digits in 1996^1996.
- Using Modular Arithmetic to Find Remainders [10/21/2004]
What is the remainder when 2^(2^405) is divided by 23?
- Using Modular Arithmetic to Test Divisibility of Large Numbers [08/30/2008]
Prove that 55^62 - 2*13^62 + 41^62 is divisible by 182.
- Using Weighted Criteria to Make Decisions [07/18/2008]
I have three employees who have each worked different numbers of days
and learned different numbers of skills in that time. How can I decide
who is the most effective employee by weighting those two factors?
- USSR Math Olympiad Puzzle [04/16/2003]
Prove that no matter what string you start with, the letters at
the corners of the triangle are either all the same or all different.
To what other numbers could you change the 'string of 10 letters' and
still have the assertion be true?
- Variable Within and Outside an Exponent [07/29/1997]
Solve for t: d = a*t + b*e^-(c*t) where a, b and c are constants and e is
- Was Euler wrong? 2*Pi=0? [03/13/2002]
While I was surfing the Internet, I found a site with an interesting
proof that shows that 2*Pi = 0 by using Euler's famous equation...
- A Way to Think about the Locker Problem [01/16/2004]
Imagine there is an endless string of lockers in your school. Person 1
starts at locker 1 and opens every locker. Person 2 starts at locker 2
and closes every 2nd locker. Person 3 starts at locker 3 and changes
every 3rd locker. Person 4 starts at locker 4 and changes every 4th
locker. Person x starts at locker x and changes every xth locker.
Which lockers are left open in rows of 25, 100, and 500 lockers?
- What Exactly is a Fraction? [10/15/2001]
What is a fraction? Is 3/1 a fraction? Is 5/sqrt(2) a fraction?
- What is 0^0? [11/01/2001]
We are doing exponents in school and we were talking about how 9^0=1,
10^0=1, etc., and I asked what 0^0 is.
- What is 0.999... + 0.999...? [09/27/1999]
How can I show that 0.999... + 0.999... = 2?
- What is Modulus? [06/06/2001]
I have used the mod command and know what the results mean, but I don't
understand the theory behind it and what is actually happening.
- What is Octal? [11/30/1999]
What is octal?
- What is the Gamma Function? What is Gamma of 4? [05/28/1998]
Deriving G(4) = 3! from the gamma function integral.
- What is the Largest Named Number? [12/08/2004]
A discussion of various large numbers and how they are defined,
including google, googleplex, and Moser, Skewes, and Graham numbers.
- What Kind of Number is One? [12/06/2001]
What is one, if it is neither prime nor composite?
- What Type of Number Are Zero and One? [11/08/2004]
If 1 and 0 are not considered prime or composite numbers, then what
- When Casting Out Nines Fails [11/29/2001]
To prove that casting out 9's worked, I intentionally used an incorrect
quotient, but was shocked when the problem checked out correctly.
- Which Fractions Repeat? [07/21/1998]
How do you know whether a fraction will be a repeating or terminating
decimal? If repeating, how many decimal places?