# What is the square of 5 1

What is a square number?
A square number is a number that can be written as the product of two equal factors from natural numbers.
Formulas: k = a * a = a² (k and a stand for natural numbers.)
After that, a number like 4/9 = (2/3) ² is excluded here.

Conversely, a square number is created when you multiply a natural number by itself.
Formulas: a * a = a² = k (k and a stand for natural numbers.)
The same factor is called the base number.

These are the first 100 square numbers. The name is explained by the illustration of the square numbers: One finds out: Only the unit digits 0, 1, 4, 5, 6 or 9 appear in the table.
Here is a consideration:
For example, you first pick out the three-digit numbers. They can be represented as 100c + 10b + a, where a, b and c are single-digit numbers.
We have (100c + 10b + a) ² = [(100c + 10b) + a] ² = (100c + 10b) ² + 2 (100c + 10b) a + a² = 100 (10c + b) ² + 10 (20c + 2b) a + a²
= 10 {[10 (10c + b) ²] + (20c + 2b) a} + a².
This means that the square of a three-digit number can be written in the form 10x + a². The one digit of the square is determined solely by a², and these are 0, 1, 4, 9 or the last digits of 16, 25, 36, 49, 64 and 81.

These considerations can be applied to all multi-digit square numbers.

square root Top
Finding a square number is easy. It is more difficult to find the base number for a square number.
This process is called "pulling the root" or square root.

Determining a square root is no longer a problem today.
You type the number into a calculator and press the root key: 17424 - root key - 132.

Further methods are discussed using the example sqrt (17424).
 It is well known that instead of sqrt (17424) one also writes .........................................................................

1 Determination via an interval nesting
The number must be between 100 and 200 (100² = 10000,200² = 40000)
The number is between 130 and 140 (130² = 16900 and 140² = 19600)
The ones digit of 17424 is 4. Then only 132 and 138 are possible.
The following applies: sqrt (17424) = 132.

2 Determination of a factorization
You split the number into factors and develop the square number from this.
17424 = 8 * 2178 = 16 * 1089 = 16 * 9 * 121. Then the square root is 4 * 3 * 11 = 132.

3 school method from bygone times
In terms of form, too, it is a kind of written division.
1
 ... ... Divide the number from the right into pairs of digits. 2
 ... ... Find the square number that is less than or equal to the pair on the left. This is trivially 01 here. Subtract the square number. The difference is 0. 3
 ... ... Get down the next pair of numbers similar to writing dividing. In an additional calculation, divide 074 by ten times the doubled number in the top right of the first line. That number is 20. The quotient is 3. The remainder in the division is of no interest. 4
 ... ... The ancillary calculation above is usually done in your head. Add the number 3 you found to the last divisor (20), and you get 23. Write the 3 of the additional calculation in the first line. 5
 ... ... As with the written division, form 3 * 23, write the product on the next line and subtract. It results in 5. 6
 ... ... Get the next pair down, 24. In an additional calculation, divide the resulting number 524 by ten times the double number 13 in the first line. That is 260. It results in 2, a remainder remains. 7
 ... ... Write the two on the first line on the right and add it to 260. Divide the number 524 below by 262 as you would when dividing in writing. Find the difference. It is zero.So sqrt (17424) = 132 applies. The explanation of this method results from the "trinomic formula" (a + b + c) ² = a² + b² + c² + 2ab + 2ac + 2bc.
 ... ... ... ... If one sets a = 100, b = 30 and c = 2,so you get the numerical example.
You can find more on the pages of Andreas Göbel and Tino Hempel (URL below).

Before calculators existed, this method was the standard method in school and was practiced extensively. Today it is still listed as a one-off in some textbooks.
(5, page 47 "Root extraction by hand")

4 Determination according to Newton's method (or Heron method)
In this method, the convergent sequence x1= 1 and xk= (xk-1+ n / xk-1) / 2 the root x = sqrt (n) is determined more and more precisely.

It may be interesting for the younger generation to see how the procedure was programmed and illustrated in the past. The predominant programming language in the 1970s was BASIC. 10 REM *** NEWTON'S APPROXIMATION20 REM FOR DETERMINING THE SQUARE ROOT ***30 INPUT Y, E40 LET A = 150 LET B = Y / A60 LET M = .5 * (A + B)70 IF ABS (A-B) 80 LET A = M90 GOTO 50100 PRINT M110 END (6, p. 92f.)
You are amazed: only after eleven steps does the root 132 result.
The reason is the unfavorable initial value A = 1. It should be as close as possible to the root.
The "Heron Approach" is A = Y.

consequences Top
There are five infinite sequences associated with square numbers. These are
> the sequence of natural numbers,
> the sequence of the square numbers,
> the sequence of the partial sums of the square numbers, i.e. the series of square numbers,
> the sequence of the reciprocal values ​​of the square numbers,
> the series of reciprocal values ​​of the square numbers.

To the first and second episode From the first two sequences one could see that the number of square numbers is smaller than the number of natural numbers. But this statement is wrong. There is no such thing as an "infinite" number. One could rather say that there are equal numbers of natural and square numbers, because one can clearly assign a square number to every natural number. One speaks more precisely of equal or equivalent and refers with these expressions to the set of natural and square numbers.
It cannot be overlooked that up to a certain number there are more natural numbers than square numbers. This statement is grasped in a first approximation to the problem by a way of speaking the set theory:
The set of square numbers is a real subset of the set of natural numbers.

To the second episode
The sequence of the square numbers is also known as the arithmetic sequence of the second order. If one forms the differences between two successive terms of the sequence, an arithmetic sequence results. It has the property that the difference is constant. The sequence of the square numbers thus has constant "differences of differences".
The formula for this is qn-qn-1= 2n-1.

To the third episode The second line is obtained from the square numbers by adding up:
s1=1²=1
s2=1²+2²=5
s3=1²+2²+3²=14
s4=1²+2²+3²+4²=30, ...
In general, sn= 1² + 2² + 3² + ... + n² = n (n + 1) (2n + 1) / 6.

Possible derivation
 Approach: sn= An³ + Bn² + Cn s1= 1 results in 1 = A + B + Cs2= 5 gives 5 = 8A + 4B + Cs3= 14 gives 14 = 27A + 9B + 3C It followsA = 1/3, B = 1/2, C = 1/6
Then sn= (1/3) n³ + (1/2) n² + (1/6) n = n (n + 1) (2n + 1) / 6.
(4, page 28f.)

To the fourth and fifth episodes
The fourth episode is 1/1, 1/4, 1/9, 1/16, ... , 1 / n². It has the limit value 0 for n towards infinity.
The associated infinite series 1/1 + 1/4 + 1/9 + 1/16 + is interesting, ...
It is convergent and has the limit value pi² / 6 (according to Euler) or rounded 1.65.
(3, page 93f. And page 241)

The corresponding series formed from the prime numbers, namely "Sum of 1 / p", is divergent. From this one concludes that the prime numbers are closer than the square numbers (2, page 322).

to hum Top
Sum of the odd numbers
The sum of the first odd numbers is a square number. More precisely, 1 + 3 + 5 + ... + (2n-1) = n².
Example: 1 + 3 + 5 + 7 + 9 = 5².

Difference between two square numbers
Every prime number p> 2 can be clearly represented as the difference between two square numbers.
Example: 37 = 361-324 = 19²-18²

Division by 8
The squares of the odd numbers, when divided by 8, leave the remainder 1.
Example: 19² : 8=361 : 8=45+1/8

Lagrange's theorem
Every natural number n can be represented as a sum of at most four square numbers.
Example: 85 = 64 + 16 + 4 + 1

Fermat-Euler's prime number theorem or Fermat's two-squares theorem
A prime number of the form 4n + 1 must be represented as the sum of two squares.
Examples: 5 = 1² + 2², 13 = 2² + 3², 625 = 7² + 24² = 15² + 20²

Pythagorean numbers
There are triples of numbers that satisfy the formula a² + b² = c². You can find out more about it on my website 3-4-5 triangle read more.

Numbers games Top
In this chapter, among other things, results from Lietzmann's book (7) from 1948 are compiled and supplemented with computer help.

Calculation from 51² to 59²
Example: 56² = 3136
(50 + E) ² = (25 + E) 100 + E²
Implementation: Add 25 to the ones digit and append the square of the ones digit.

Calculation of three-digit numbers with a 0 as a tens digit
Example: 203² = 41209, 609²=370881
(H0E) ² = H² * 10000 + 2U * 100 + E²
Implementation: The two spaces on the outside are formed by the squares of the hundreds digit and the ones digit. In the middle is the double product of hundreds and ones. If the double product has three digits, then the carryover 1 must be added to the square of the hundreds.

Squares from the digits 0 to 9. Each digit occurs exactly once.
There are 77 numbers.
1026 753 849 = 32043² is the smallest number.
9814072356 = 99066² is the largest number.
The ten-digit numbers from different digits are called "Pandigital numbers" in English.

Square numbers from the digits 1 to 9. Each digit occurs exactly once.
Two examples: 139845276 = 11826² and 923187456 = 30384²
The nine-digit numbers from the digits 1 to 9 are called "zeroless pandigital numbers" in English.

Mirror numbers
 122 = 144 and 212 = 44113² = 169 and 31² = 961 102² = 10404 and 201² = 40401103² = 10609 and 301² = 90601112² = 12544 and 211² = 44521113² = 12769 and 3112 = 96721 1012² = 1024144 and 2101² = 44142011112² = 1236544 and 2111² = 44563211212² = 1468944 and 2121² = 44986412012² = 4048144 and 2102² = 4418404

Consecutive numbers
 8281=91² 183184=428²328329=573²528529=727²715716=846² 60996100=7810²82428241=9079²98029801=9901² 1322413225=36365²4049540496=63636²

Palindromes under the square numbers
 121=11²484=22²676=26² 10201=101²12321=111²14641=121²40804=202²44944=212²69696=264²94249=307² 698896=836² 1002001=1001²1234321=1111²4008004=2002²5221225=2285²6948496=2636² 123454321=11111²

Five sequences of square numbers
 1²=111²=121111²=123211111²=1234321... 4²=1634²=1156334²=1115563334²=11115556... 7²=4967²=4489667²=4448896667²=44448889... 9²=8199²=9801999²=9980019999²=999800001... 13²=169133²=176 891333²=1 77688913333²=1777 68889...

Kaprekar numbers
A number like 703 is called the Kaprekar number because it has the following strange property:
If you square the number 703² = 494209 and divide the square into 494 and 209, the sum 494 + 209 is 703 again.

The Kaprekar numbers below 10000 are 1, 9, 45, 55, 99, 297, 703, 999, 2223, 2728, 4879, 4950, 5050, 5292, 7272, 7777 and 9999.

Figure numbers Top
The square numbers belong to the figure numbers.

 Triangular numbersSquare numbersPentagonal numbersHexagonal numbersHeptagon numbersOctagonal numbers... n * (n + 1) / 2n²n * (3n-1) / 2n * (4n-2) / 2n * (5n-3) / 2n * (3n-2)... 1 3 6 10 15 21 28... 1 4 9 16 25 36 49...1 5 12 22 35 51 70... 1 6 15 28 45 66 91... 1 7 18 34 55 81 112... 1 8 21 40 65 96 133......
One problem is to find out which square numbers appear in the remaining number sequences.

At Mathworld you can find
Square numbers under the triangular numbers: 1, 36, 1225, 41616, 1413721, 48024900, ... Sloane's A001110
Square numbers among the pentagonal numbers: 1, 9801, 94109401, 903638458801, 8676736387298001, ... (Sloane's A036353)

More about square numbers can be found elsewhere on my homepage.

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Square numbers on the internet Top

German

Andreas Göbel
Pulling a root - paper pencil method

Math prism
Square numbers, number acrobatics

Tino Hempel
The written drawing of a square root

Wikipedia
Square number, Square root, Centered square number, Root (mathematics), Written root extraction,
Four squares set

English

Eric W. Weisstein (MathWorld)
Square Number, Squarefree, Squareful, Antisquare Number, Square Triangular Number, Pentagonal Square Number,
Heptagonal Square Number, Octagonal Square Number, Square Root, Square Root Algorithms, Newton's Iteration

Patrick De Geest (World of Numbers)
Extraordinary squares and powers (non-palindromic allowed)

Neil J.A. Sloane
Integer sequences
The squares: A000290

Wikipedia
Square number, Polygonal_number, Triangular square number, Pandigital_number, 9814072356 (number),
Square root, Methods of computing square roots, Euler's four-square-identity, Kaprekar number,
Lagrange's four-square theorem

credentials Top
(1) Hermann Schubert: Mathematical leisure hours, Berlin 1941 (headword: "Ludendo discimus")
(2) Heinrich Behnke et al. (Ed.): Mathematik 1, DAS FISCHER LEXIKON, Frankfurt am Main 1964
(3) Jean-Paul Delahaye: Pi - the story, Basel 1999 [ISBN 3-7643-6056-9]
(4) Maximilan Miller: Solved and unsolved mathematical problems, Leipzig 1982
(5) August Schmid et al .: Lambacher / Schweizer LS9, Stuttgart 1996 [ISBN 3-12-730740-3]
(6) Forsythe, Keenan, Organick, Stenberg: Programming with BASIC, Braunschweig 1976 [ISBN 3-594-10854-6]
(7) Walter Lietzmann: Oddities in the realm of numbers, Bonn 1948

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