A Note on Gaussian Integers, Pythagorean Triples, and Extended Pythagorean Triples David Alger The Citadel Math 495 Section I – Introduction: Pythagoras is credited for coming to the conclusion that the areas of the two squares erected on the two legs of a right triangle would equal the area of the square erected on the hypotenuse. This is known as the Pythagorean Theorem. Pythagoras, who had a mystical view of integers [2], is credited with uncovering certain integer patterns which satisfied the “algebra” version of the Pythagorean Theorem, that is integers a, b, and c with a2 + b2 = c2. We are looking in this paper at these integers and also at the Gaussian Integers, that is, the set of numbers of the form a + bi in which a and b are Integers and i 1 . Gaussian Integers can also be described as the set of algebraic integers in the finite extension of field Q(i). The study is to explore the connection between Pythagorean triples and Gaussian integers. It was known to the Greeks and proved by Euler that if a number c could be factored as c = c1 * c2 where each of c1 and c2 could be written as a sum of two squares, then c could be written as a sum of two squares in two ways. Here, c1 = x2 + y2 and c2 = a2 + b2 and c = (x2 + y2)*(a2 + b2). We say (a, b, c) is a PT to mean it is a Pythagorean Triple, and if further, gcd (a,b,c) = 1, then we say (a,b,c) is a Primitive Pythagorean Triple (or PPT) Theorem 1 [Euclid’s 47th Proposition]: If (a, b, c) is a PPT, then a = 2mn, b = m2 – n2 and c = m2 + n2 for some pair of integers m, n with 1. m > n 2. gcd(m, n) = 1 3. m, n = not both odd. Many great mathematicians have had their share of successes with the Pythagorean triples. They each came up with their own rule for finding numbers that make up Pythagorean Triples. We list some of them below [3]: 1. Rule of Pythagoras: Let n be odd; then the triple (n, 2 2 n2 1 n2 1 , ) is a PPT. Notice 2 2 n2 1 4n 2 n 4 2n 2 1 n 2 1 . See Table 1. n 4 2 2 2 Plato’s Rule: Let m be any even number divisible by 4; then (m, 2. 2 m2 m2 1 ) is a 1 , 4 4 2 m2 m2 m4 m2 1 m 2 1 1 . See Table 2. PPT since m 16 2 4 4 3. Euclid’s Rule: Let x and y be any two even or odd numbers, such that x and y contain no x y , and x y are three common factor greater than 2, and xy is a square. Then xy , 2 2 2 x 2 2 xy y 2 x y x y such numbers. For xy xy . 4 2 2 m2 n2 4. Rule of Maseres: Let m and n be any two even or odd, m > n, and a square 2n 2 2 m2 n2 2 m n integer. Then m , , and are three such numbers. For 2n 2n 2 2 2 2 m 2 n 2 4m 2 n 2 m 4 2m 2 n 2 n 4 m 2 n 2 . m 2n 4n 2 2n 5. Dickerson’s Rule: Let m and n be any two prime integers, one even and the other odd, m>n and 2mn a square. Then m 2mn , n 2mn , and m n 2mn is a PT. For 2 m 2 2 2 2mn n 2mn m 2 n 2 2mn 2m 2mn 2n 2mn m n 2mn . 6. If the two legs are represented by a and b and the hypotenuse is represented by c the following relations apply when p and q are two integers and k = gcd (a, b, c): a k p2 q2 b k 2 pq c k p2 q2 When k is equal to one the triple (a, b, c) is said to be primitive. Section II –Gaussian Integers and PTs: There is a close connection with the set of Gaussian integers and the set of PT’s. In fact, manipulating the Gaussian Integers you are able to get Pythagorean Triples. If you square any Gaussian Integer you will get a new Gaussian Integer. The magnitudes of those coefficients are the lengths of the legs of a right triangle. The hypotenuse is found by taking the norm of the Gaussian Integer. The norm of a Gaussian Integer x yi is x 2 y 2 . For example, using a randomly selected z 3 2i , we see z 2 5 12i , and the length of z squared is 13. But then (5,12,13) is a PT. We define, for any complex number z a bi , Re( z ) a , Im( z ) b . Here “Re” refers to “real” and “Im” refers to “imaginary”. L(z) denotes the length z, namely Norm(z) . Alger 2 Theorem 2: If z a bi is any Gaussian Integer, then ( a 2 b 2 , 2ab , a 2 b 2 ) is a PT. Proof 2: The proof is immediate applying Theorem 1. We can restate the theorem as: if z a bi is any Gaussian Integer, then ( Re( z 2 ) , Im( z 2 ) , L(z ) 2 ) is a PT. If gcd a, b 1 , then the PT is a PPT. Section III – Primes in the ring of Gaussian Integers In addition to being defined slightly differently than normal integers, Gaussian integers also have their own set of prime numbers. A Gaussian Prime can be defined as a number which can not be factored as a product of two Gaussian Integers except by using a unit {±1, ±i}. For example, since 13 32 2 2 3 2i 3 2i we see 13 is not a Gaussian Prime even though it is a prime in Z. Definition: An element α in G = {a + bi : a,b Z}is prime iff α is not a unit and whenever α = xy (for x,y G) either x or y is a unit. The factorization for 13 is an example of a serious difficulty. Note that a 2 b 2 can always be factored in G as a 2 b 2 a bi a bi . This shows two is not a prime either since 2 12 12 1 i 1 i . Since neither 1 i nor 1 i are units, 2 (by the definition) can not be a Gaussian prime. Theorem 3: No rational prime p is a Gaussian prime if p a 2 b 2 for some rational integers a, b. Proof 3: The difficult question is this: which rational primes can be written as a sum of two squares. One may also ask: what rational integers can be written as a sum of two squares? 2n 2 2m2 4n 2 4m 2 4n 2 m 2 , a multiple of 4. 2n 2 2m 12 4n 2 4m 2 4m 1 4k 1 1mod 4 2n 12 2m 12 4n 2 4n 1 4m 2 4m 1 4 j 2 These show that a 2 b 2 is 0 mod 4 , 1mod 4 , or 2 mod 4 . But a 2 b 2 3mod 4 . The point for this paper is that a rational prime q which satisfies q 3mod 4 might be a Gaussian prime – but no other rational primes can possibly be Gaussian primes. Numbers that are rational prime integers are not necessarily Gaussian prime integers and vice versa. The Gaussian primes are those rational primes that can not be expressed by the sum of two squares. In summary, the definition is saying is that a number a + bi (b ≠ 0) is prime in G if its norm is prime in Z. N(xy) = N(x)N(y) = prime (then N(x) or N(y) = 1) and so x or y is a unit. The following are two simple characteristics that would classify a number as a prime in G: 1. Primes in Z which are 3 mod 4. 2. x = a + bi for N(x) = prime. Alger 3 Section IV - Extending the Pythagorean Triples: When taking multiple triples together an extended version of a Pythagorean equation may be made. For example: 3 2 4 2 5 2 and 5 2 12 2 13 2 therefore 32 4 2 12 2 132 . This can be broken down into the following general equations: a 2 b 2 c 2 and c 2 d 2 e 2 therefore a 2 b 2 d 2 e 2 . a 3 5 7 9 Extended Pythagorean Triples b c d 4 5 12 12 13 84 24 25 312 40 41 840 e 13 85 313 841 This property can be used when putting two right triangles adjacent to each other in order to find the side lengths for larger polygons. Two examples 12 2 2 2 2 3 2 and 2 2 3 2 6 2 7 2 suggested a pattern which led to the infinite set of EPTs in Table 3 of the Appendix. Theorem 4: Suppose (n, n+1, n(n+1), n(n+1)+1) is an EPT Proof 4: If it is an EPT then n2 + (n + 1)2 + (n(n + 1))2 = (n(n + 1) + 1)2 n2 + (n + 1)2 + (n2 + n) = (n2 + n + 1)2 n2 + n2 + 2n + 1 + n4 + 2n3 + n2 = n4 + 2n3 + 3n2 + 2n + 1 Since this is true the set of numbers (n, n+1, n(n+1), n(n+1)+1) is an EPT. It is possible to turn many familiar algebra identities into new sources of EPTs. We 2 illustrate the idea with 1 2n n 2 n 1 in the next theorem. Theorem 5: Suppose 2n is a square. Then (1, 2n , n, n 1 ) is an EPT. Proof 5: In order for (1, 2n , n, n 1 ) to be an EPT the following relationship must exist. 12 + 2n 2+ n2 = ( n 1 )2 Since 12 + 2n 2+ n2 = 12 + 2n+ n2 = ( n 1 )2 the following relationship is an EPT. In order to construct more “formula based EPTs the only thing really necessary is to construct an equality with one side of the equations equaling a three term polynomial (ie 3 2 4 2 12 2 13 2 ) and the square root of each term equaling an integer. For example, taking the equality a 2 b 2 c 2 x 2 and manipulating one or two of the variables you are able to make more generalized equations. By setting a equal to a constant and x equal to a modification Alger 4 of c you could get a whole set of answers. By varying the constant you will get different sets of answers. The following theorem will demonstrate one of those sets. Theorem 6: Suppose the equation a 2 b 2 c 2 x 2 is true. Set a equal to a constant and x equal to c + a. Proof 6: By substitution you see that 12 b 2 c 2 (c 1) 2 . So 1 b 2 c 2 c 2 2c 1 and the c2 terms and the ones cancel out to get b 2 2c 1 . By simplifying and solving b2 for c you get it to be equal to . After putting that in a table you realize that as long as 2 a is equal to one and b is an even number you will always get an EPT. See Table 4. By generalizing that theorem even more and testing for different values of the constant we come up with an Über theorem as follows: Über Theorem: Suppose the equation a 2 b 2 c 2 x 2 is true. By setting a equal to a constant b2 and x set equal to c + a, then c will be equal to . If a is an even number then a EPT 2a will be formed for every value of b equal to a*n. If a is an odd number then a EPT will be formed for every value of b equal to 2*a* n. Where n Z. Über Proof: By substitution the equation becomes a 2 b 2 c 2 (c a) 2 c 2 2ac a 2 . By b2 simplifying we get b 2 2ac and by further substitution b 2 2a we show that all 2a terms cancel out so the equation is in fact equivalent. See Table 5 for examples of various constant values. Alger 5 Appendix: n 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 n 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Table 1 (n -1)/2 (n2+1)/2 4 5 12 13 24 25 40 41 60 61 84 85 112 113 144 145 180 181 220 221 264 265 312 313 364 365 420 421 480 481 544 545 612 613 684 685 760 761 840 841 2 n+1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Table 3 n(n+1) 2 6 12 20 30 42 56 72 90 110 132 156 182 210 240 272 306 342 380 420 m 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 n(n+1)+1 3 7 13 21 31 43 57 73 91 111 133 157 183 211 241 273 307 343 381 421 a 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Alger 6 Table 2 m2/4-1 m2/4+1 3 5 15 17 35 37 63 65 99 101 143 145 195 197 255 257 323 325 399 401 483 485 575 577 675 677 783 785 899 901 1023 1025 1155 1157 1295 1297 1443 1445 1599 1601 b 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Table 4 c = b2/2 2 8 18 32 50 72 98 128 162 200 242 288 338 392 450 512 578 648 722 800 x=c+1 3 9 19 33 51 73 99 129 163 201 243 289 339 393 451 513 579 649 723 801 n a 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 Table 5 b 2 4 6 8 10 12 14 16 18 20 2 4 6 8 10 12 14 16 18 20 6 12 18 24 30 36 42 48 54 60 4 8 12 16 20 24 28 32 36 40 10 20 30 40 50 60 70 80 90 100 c x 2 8 18 32 50 72 98 128 162 200 1 4 9 16 25 36 49 64 81 100 6 24 54 96 150 216 294 384 486 600 2 8 18 32 50 72 98 128 162 200 10 40 90 160 250 360 490 640 810 1000 3 9 19 33 51 73 99 129 163 201 3 6 11 18 27 38 51 66 83 102 9 27 57 99 153 219 297 387 489 603 6 12 22 36 54 76 102 132 166 204 15 45 95 165 255 365 495 645 815 1005 Alger 7 Works Cited: [1] A. H. Beiler, Recreations in the Theory of Numbers, Second Edition, Dover Publications, INC., New York, 1966. [2] D. M. Burton, The History of Mathematics, Allyn and Bacon, INC., Boston, Massachusetts, 1985. [3] E. S. Loomis, The Pythagorean Proposition, Second Edition, National Council of Teachers of Mathematics, Ann Arbor, Michigan, 1940. Alger 8