Square number

In mathematics, a square number, sometimes also called a perfect square, is an integer that is the square of an integer;[1] in other words, it is the product of some integer with itself. So, for example, 9 is a square number, since it can be written as 3 × 3.

The usual notation for the formula for the square of a number n is not the product n × n, but the equivalent exponentiation n2, usually pronounced as "n squared". The name square number comes from the name of the shape. This is because a square with side length n has area n2.

Square numbers are non-negative. Another way of saying that a (non-negative) number is a square number, is that its square root is again an integer. For example, √9 = 3, so 9 is a square number.

A positive integer that has no perfect square divisors except 1 is called square-free.

For a non-negative integer n, the nth square number is n2, with 02 = 0 being the zeroth square. The concept of square can be extended to some other number systems. If rational numbers are included, then a square is the ratio of two square integers, and, conversely, the ratio of two square integers is a square (e.g., 4/9 = (2/3)2).

Starting with 1, there are \lfloor \sqrt{m} \rfloor square numbers up to and including m, where the expression \lfloor x \rfloor represents the floor of the number x.

Contents

Examples

The squares (sequence A000290 in OEIS) below 602 are:

02 = 0
12 = 1
22 = 4
32 = 9
42 = 16
52 = 25
62 = 36
72 = 49
82 = 64
92 = 81
102 = 100
112 = 121
122 = 144
132 = 169
142 = 196
152 = 225
162 = 256
172 = 289
182 = 324
192 = 361
202 = 400
212 = 441
222 = 484
232 = 529
242 = 576
252 = 625
262 = 676
272 = 729
282 = 784
292 = 841
302 = 900
312 = 961
322 = 1024
332 = 1089
342 = 1156
352 = 1225
362 = 1296
372 = 1369
382 = 1444
392 = 1521
402 = 1600
412 = 1681
422 = 1764
432 = 1849
442 = 1936
452 = 2025
462 = 2116
472 = 2209
482 = 2304
492 = 2401
502 = 2500
512 = 2601
522 = 2704
532 = 2809
542 = 2916
552 = 3025
562 = 3136
572 = 3249
582 = 3364
592 = 3481


The difference between any perfect square and its predecessor is given by the identity n ^ 2 = (n - 1) ^ 2 %2B (2 n - 1) . Equivalently, it is possible to count up square numbers by adding together the last square, the last square's root, and the current root, that is, n ^ 2 = (n - 1) ^ 2 %2B (n - 1) %2B n .

Properties

The number m is a square number if and only if one can arrange m points in a square:

m = 12 = 1
m = 22 = 4
m = 32 = 9
m = 42 = 16
m = 52 = 25

The expression for the nth square number is n2. This is also equal to the sum of the first n odd numbers as can be seen in the above pictures, where a square results from the previous one by adding an odd number of points (shown in magenta). The formula follows:

n^2 = \sum_{k=1}^n(2k-1).

So for example, 52 = 25 = 1 + 3 + 5 + 7 + 9.

There are several recursive methods for computing square numbers. For example, the nth square number can be computed from the previous square by n^2 = (n-1)^2 %2B 2n-1. Alternatively, the nth square number can be calculated from the previous two by doubling the (n − 1)-th square, subtracting the (n − 2)-th square number, and adding 2, because n2 = 2(n − 1)2 − (n − 2)2 + 2. For example, 2 × 52 − 42 + 2 = 2 × 25 − 16 + 2 = 50 − 16 + 2 = 36 = 62.

A square number is also the sum of two consecutive triangular numbers. The sum of two consecutive square numbers is a centered square number. Every odd square is also a centered octagonal number.

Another property of a square number is that it has an odd number of divisors, while other numbers have an even number of divisors. An integer root is the only divisor that pairs up with itself to yield the square number, while other divisors come in pairs.

Lagrange's four-square theorem states that any positive integer can be written as the sum of four or fewer perfect squares. Three squares are not sufficient for numbers of the form 4k(8m + 7). A positive integer can be represented as a sum of two squares precisely if its prime factorization contains no odd powers of primes of the form 4k + 3. This is generalized by Waring's problem.

A square number can end only with digits 0,1,4,6,9, or 25 in base 10, as follows:

  1. If the last digit of a number is 0, its square ends in an even number of 0s (so at least 00) and the digits digits preceding the ending 0s must also form a square.
  2. If the last digit of a number is 1 or 9, its square ends in 1 and the number formed by its preceding digits must be divisible by four.
  3. If the last digit of a number is 2 or 8, its square ends in 4 and the preceding digit must be even.
  4. If the last digit of a number is 3 or 7, its square ends in 9 and the number formed by its preceding digits must be divisible by four.
  5. If the last digit of a number is 4 or 6, its square ends in 6 and the preceding digit must be odd.
  6. If the last digit of a number is 5, its square ends in 25 and the preceding digits must be 0, 2, 06, or 56.

In base 16, a square number can end only with 0,1,4 or 9 and
- in case 0, only 0,1,4,9 can precede it,
- in case 4, only even numbers can precede it.

In general, if a prime p divides a square number m then the square of p must also divide m; if p fails to divide m/p, then m is definitely not square. Repeating the divisions of the previous sentence, one concludes that every prime must divide a given perfect square an even number of times (including possibly 0 times). Thus, the number m is a square number if and only if, in its canonical representation, all exponents are even.

Squarity testing can be used as alternative way in factorization of large numbers. Instead of testing for divisibility, test for squarity: for given m and some number k, if k² − m is the square of an integer n then k − n divides m. (This is an application of the factorization of a difference of two squares.) For example, 100² − 9991 is the square of 3, so consequently 100 − 3 divides 9991. This test is deterministic for odd divisors in the range from k − n to k + n where k covers some range of natural numbers k ≥ √m.

A square number cannot be a perfect number.

The sum of the series of power numbers

\sum_{n=0}^N n^2 = 0^2 %2B 1^2 %2B 2^2 %2B 3^2 %2B 4^2 %2B \cdots %2B N^2

can also be represented by the formula

\frac{N(N%2B1)(2N%2B1)}{6}.

The first terms of this series (the square pyramidal numbers) are:

0, 1, 5, 14, 30, 55, 91, 140, 204, 285, 385, 506, 650, 819, 1015, 1240, 1496, 1785, 2109, 2470, 2870, 3311, 3795, 4324, 4900, 5525, 6201... (sequence A000330 in OEIS).

All fourth powers, sixth powers, eighth powers and so on are perfect squares.

Special cases

Odd and even square numbers

Squares of even numbers are even (and in fact divisible by 4), since (2n)2 = 4n2.

Squares of odd numbers are odd, since (2n + 1)2 = 4(n2 + n) + 1.

It follows that square roots of even square numbers are even, and square roots of odd square numbers are odd.

Uses

Since the product of real negative numbers is positive, and the product of two real positive numbers is also positive, it follows that no square number is negative. It follows that no square root can be taken of a negative number within the system of real numbers. This leaves a gap in the real number system that mathematicians fill by postulating complex numbers, beginning with the imaginary unit i, which by convention is one of the square roots of −1.

Squaring is used in statistics in determining the standard deviation of a set of values. The deviation of each value x_i\, from the mean \overline{x} \, of the set is defined as the difference x_i - \overline{x} \,. These deviations are squared, then a mean is taken of the new set of numbers (each of which is positive). This mean is the variance, and its square root is the standard deviation. In finance, the volatility of a financial instrument is the standard deviation of its values.

See also

Notes

  1. ^ Some authors also call squares of rational numbers perfect squares.

References

Further reading

External links