# Factoring Patterns in the Gaussian Plane

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1 Factoring Patterns in the Gaussian Plane Steve Phelps Introduction This paper describes discoveries made at the Park City Mathematics Institute, 00, as well as some proofs. Before the summer I understood Apollonian circles, inversion, complex number, and the like and had a frame of reference when trying to find patterns in the factors. When first looking for patterns, I noticed them because of my prior knowledge. Without that prior knowledge, I might not have discovered the patterns shared here. Some things here are accessible to high school students, but others are stated without proof but with references for the interested reader. 00 morning problem solving sessions focused on the Gaussian integers. Geometrically, Gaussian integers represent the lattice points of the Cartesian coordinate plane as in Figure i -3+i i i - Figure. The Gausian Integers are the lattice points in the coordinate plane Algebraically, the Gaussian integers are a subset of the complex numbers. Symbolically, let Gbe the set of Gaussian integers. Then { a ib : a, b and i } G = + =.

2 One goal of the morning problem solving sessions was to determine those integers that can be written as the sum of two squares. For example, 3 = + 3. However, 7 cannot be written as the sum of two squares. Furthermore, some numbers can be written as the sum of two squares in more than one way. Observe that 0 = 7 + = +. The key to determining which numbers could and could not be written as the sum of two squares was the multiplication and factorization of Gaussian integers. Multiplying Gaussian Integers The nature of some morning activities led to the multiplication of Gaussian integers. For those familiar with multiplying complex numbers, the ordinary complex multiplication will do: For those less familiar, a hiking analogy is appropriate. To explore the effects of multiplying by ordinary integers or by integer multiples of i, pretend you are standing at the origin of a graph, facing in the direction of. If multiplying by an ordinary integer, say, simply step off two + 3i steps in the direction of + 3i. If multiplying by, take two + 3i steps backwards. If multiplying by i before taking any steps, turn 90 to the left, then take one + 3i step. If multiplying by i, turn 90 to the left, then take two the same effect, except requires turning Figure. + 3i ( a + bi)( c + di) = ( ac bd) + i( ad + bc) + 3i + 3i steps. Multiplying by negative integer multiples of i has 90 to the right before taking any steps as in Multiples of +3 6 (+3i) i(+3i) i(+3i) +3i (+3i) - -i(+3i) - -i(+3i) -(+3i) -8 Figure. Some multiples of + 3i

3 Figure 3 shows more Gaussian integer multiples of. Plotting all the multiples has the effect of creating a lattice-sort of road map showing how to get from the origin to any other multiple of + 3i. The lattice can also be obtained by rotating the coordinate plane about the origin through an angle of θ = tan radians, followed by a dilation centered at the origin with a scale factor of that it shows how to get from the origin to any multiple of + 3i i 3. The essence of this lattice is (-+3i)(+3i) 3i(+3i) (-+i)(+3i) (+i)(+3i) i(+3i) 8 6 (+i)(+3i) i(+3i) (+i)(+3i) +3i (3-i)(+3i) (+3i) (-i)(+3i) (-i)(+3i) (-i)(+3i) (-+i)(+3i) 0(+3i) -0-0 (-+i)(+3i) -(+3i) (-+i)(+3i) -(+3i) (-3+i)(+3i) - - (--i)(+3i) (-i)(+3i) -i(+3i) (-3i)(+3i) -i(+3i) -3i(+3i) (--i)(+3i) -8 Finding other products Figure 3. A lattice of multiples of One challenge is to find more than one way to express a Gaussian integer as the product of two Gaussian integers. There is a correct way to do this and an incorrect way ( )( ) to do this. For example, 3 = + 3i 3i ( 3+ i)( 3 i) + 3i. However, 3 can also be written as. This would not count as a different way of expressing 3. The factors used are called associates. Associates are two Gaussian integers who differ only by a factor of -. For example, notice that. We are looking for

4 two products whose factors are not associates. For example, and ( i)( i) 0 = But how does one find these factors? Guess and check could certainly be a method one might employ (but not a very efficient one). A different method of finding factors is based on the geometry of the multiplication algorithm, and on the geometry of the circle, particularly, the fact that a right angle can be inscribed in a semi-circle. A Method for Factoring Integers The following described method for factoring integers is based on circles and right angles. The right angles are found in the multiplication of Gaussian integers (multiplying by i or i). To illustrate this method, we find the Gaussian factors of 0. In Figure, stand at the origin facing + 3i, and take one + 3i step forward (multiplying by ). Turn to the right (multiplying by i) and take three + 3i steps forward (multiplying by 3i). The result is 0. In total, + 3i is multiplied by followed by 3i, or using the distributive property, + 3i 3i = 0. ( )( ) 3 +3i -i ( +3i) ( ) - 3 i = 0 -i -i Figure. One way to factor 0 It is easy to find this factorization of 0. But are there others? In Figure, notice the right angle at + 3i. From elementary geometry, there is a circle through + 3i, the origin and 0. Furthermore, this circle contains nine other lattice points other than 0 and

5 the origin as seen in Figure. Each one of these lattice points can be used to factor 0 into two Gaussian factors. Associated with each lattice point is a right triangle whose hypotenuse is the segment from the origin to 0 as in Figure 6. In practice, one need not work with all the triangles in Figure 6, only the triangles in the first quadrant. 3 +3i +i +i 8+i 9+3i i 9-3i - - -i -i 8-i Figure. The lattice points on the circle

6 6 3 +3i +i +i 8+i 9+3i i 9-3i - - -i -i 8-i Figure 6. The right triangles associated with each lattice point A specific example: Factoring 0 using the lattice point +i Let s explore the right triangle created by the lattice point associated with + i triangle associated with + i, + 3 i, 6 + i, and 8 + i determine the factors of 0. (the triangle ) and see what factors that lattice point reveals. In Figure 7, the contains four other lattice points:. These lattice points on the triangle can be used to Stand at the origin and face the lattice point be to another lattice point on the triangle in the direction of. Taking a step requires that it. Furthermore, every step must be the same size and requires planning. Standing at the origin, there are two choices for the first step: + i, or + i, leading half way to the lattice point + i. Standing at the origin facing the lattice point + i, take two steps (multiplying by ). That leads to + i. Standing at + i, turn 90 to the right (multiplying by i) and take four + i steps (multiplying by i). That leads to 0. In total, + i by, followed by i. Using the distributive property, ( i)( i) + = 0. + i + i + i + i

7 i +3i 6+i +i 8+i Figure 7. Factoring + i What happens if we start out taking one step? That step leads to the lattice point. Turning 90 to the right (multiplying by ) and taking two + i steps (multiplying by i), leads 0. Using the distributive property, + i i = 0. Though the factorization method is essentially the same as the other one (the factors are associates), it nevertheless is a factorization, and the lattice point led to four factors of 0: + i, i, + i, and i. Is it true that each lattice point on the circle is a factor of 0? No. Referring back to Figure 6, for9 + 3i to be a factor, one needs to take a whole number of 9 + 3i origin and taking one that is a multiple of steps along both legs of the associated right triangle. Standing at the step leads to the lattice point. Turning right requires a step which is impossible. + i ( )( ) 9 + 3i 9 + 3i The factors of 0 revealed by each lattice point are seen in Table. Readers should verify that these are indeed factors (not by multiplying, but by employing the methods of this article). For those familiar with the workings of complex numbers, the factors occur in conjugate pairs a ± ib. And of course, these are not the only Gaussian factors of 0;,,, and 0 are left off the list for now. Not because they are not Gaussian integers, for they are. Each is of the form a + bi with b = 0. The list of factors

8 can be expanded by considering all the associates of each factor. For example, if a factor of 0, then so are + i, i and i. Using only the factors in Table, What other patterns are there in the Gaussian factors of 0? Lattice Points Table. Factors from the lattice points Factors + i + i, i, i, + i 8 + i + i, i, + i, i 9 + 3i 3 + i,3 i + i + i, i, + i, i + 3i + 3 i, 3i + i is Patterns in the Gaussian Factors of Figure 8. The Gaussian Factors of 0

9 Seeing all the Gaussian factors of 0 as in Figure 8) should be convincing that there is indeed some sort of pattern or structure to the factors that begs for further study. There is obviously rotational symmetry and reflection symmetry thanks to the associate factors. Is there any other symmetry or structure that is hidden in the factors? Pursuing this question is where things really get interesting. point Step back for a moment and examine the factors of 0 associated with the lattice + i. Graphing them, there seems to be nothing exceptionally interesting except for the reflection symmetry in the x-axis due to the conjugate pairs seen in Figure 9. However, with an understanding of elementary geometry and knowledge of quadrilaterals, these factors are recognized as the vertices of an isosceles trapezoid. Isosceles trapezoids are cyclic, which means their vertices lie on a circle. So the factors of 0 associated with the lattice point lie on a circle. Now, three non-linear random points are all that are needed to determine a unique circle. The probability that four random points be contained in one circle is rather small (in fact, the probability is zero). Granted, these may not be random points, but there must be something up here for the factors to be concyclic. + i 6 +i +i 0 - -i - -i Figure 9. The factors associated with the lattice point + i lie on a circle.

10 8 + i What about the other lattice points? It is easy for the lattice points because they have two pairs of conjugate pair factors. The factors for each will lie on a circle. The factors for + 3i and 9 + 3i do lie on a circle, but they seem to break the pattern as seen in Figure 0. Verifying this is worth the effort and cements an understanding of how the factoring method works. + i and 7 C is the Circle through lattice factors for +3i and 9+3i. 6 3 C 3 is the Circle through 8+i lattice factors. c c 3 C is the Circle through +i lattice factors. c c C is the Circle through +i lattice factors. Figure 0. The factors on circles, with one circle that does not fit the pattern. And what about the ordinary integer factors,,, and 0? In Figure, these factors are shown to lie on circles, too, providing stronger evidence that there is something funny about the circle that seems to break the pattern. However, all the other circles required points! How can a circle be determined through and? The answer to this question lies in what are known as coaxal circles. After discussing these circles, we explore the circles that make up a coaxal system of circles and gain a better understanding on how to make all the factors fit the pattern. Moreover, we extend the pattern in a number of unsuspecting ways.

11 A B Figure. All the factors lie on circles, with one that does not fit the pattern! Coaxal Circles an Introduction Figure is an example of coaxal circles, or a coaxal system of circles. In Figure, there are two distinctly different families of circles one blue and the other red. The blue family of circles passes through the points A and B. The red family of circles seems to shrink towards the point A and B. Points A and B are referred to as the limiting points of the coaxal system. Furthermore, points A and B lie on the blue line. This blue line is considered to be a circle in the blue family. Consider it to be a circle of infinite radius. Likewise, there appears to be a red line perpendicular to the blue line (indeed, it is the perpendicular bisector of A and B). It, too, is considered to be a circle of infinite radius, and is a full-fledged member of the red family. Figure.

12 A Coaxal System of Circles red circle at a What is intriguing about a coaxal system is that every blue circle intersects every 90 angle. Thus, the blue circles and red circles are orthogonal. Given that every circle in one family is orthogonal to every circle in the other, it seems reasonable to consider the two perpendicular lines to be a part of their respective families. The two perpendicular lines serve as a sort of axis for the system (hence the name coaxal), in that every circle of the blue family has its center on the red perpendicular, and every circle in the red family has its center on the blue perpendicular. The red family of circles is a special family of circles known as Apollonian circles. Apollonian circles are defined as follows: given two points A and B and a point P, an Apollonian circle is the locus of points satisfying where is the distance from P to A, the distance from P to B, and k is a constant, k 0. In layman s terms, an Apollonian circle is the collection of points P whose distance from a given point A is a multiple of its distance from another given point B as seen in Figure 3. P A = k P B P A P B P-A P-B =0.9 A B P Figure 3. Some Apollonian circles with a red one for A proof that the definition of Apollonian circles (the red circles) define a family of coaxal circles In Figure, coaxal circles have centers lying on the x-axis (the red circles) or y- axis (the blue circles). Therefore, let the limit points be located at A( a,0 ) and B( a,0).

13 Furthermore, the centers of these circles will be located on the x-axis. Let point P( x, y ) be such that where d ( A, P ) means the distance from A to P and k 0. This is the definition of Apollonian circles. Using the distance formula yields ( ) ( ) ( ( )) x a + y = k x a + y or ( ) ( ) x a + y = k x + a + k y. Expanding we have x ax + a + y = k x + k ax + k a + k y. Collecting like terms we have ( ) ( ) ( ) ( ) k x a + k x + k y = k a. Dividing to make the leading coefficient we have ( + k ) ( k ) x a x + y = a Completing the square we have ( ) ( ). ( ) ( ) ( ) ( ) + k + k + k + + = x a x a y a a k k k Simplifying, we get ( k ) ( k ) ( k ) ( k ) + + x a + y = a + k Lettingλ =, the equation above becomes k ( x aλ) y a ( λ ) + =. This is the equation of a circle of radius r = a( λ ) and center ( aλ,0) circle in the red family has a radius of r = a( λ ) and center ( aλ,0)... Hence, every. We have established that the definition of Apollonian circles does indeed describe a family of circles. + k So as to understand these red circles better, examine what happens to λ = k as k varies among the all possible values for k 0. Two values of k which are of

14 interest are when k = 0 and when k = (this is a zero of the denominator). When k = 0, λ =. We glean two bits of information about the circles. First of all, by the definition of Apollonian circles, d ( P, B) k d ( P, A) d ( P, B ) = 0 = ; therefore,. Hence, point P and point B are the same point. Second of all, when k = 0, λ =, and that corresponds to a circle centered at ( a,0) of radius r = 0. Again, this means point P and point B are the same point. Hence, when λ = limiting points of the system., P is one of the When k =, λ is undefined (but λapproaches as k approaches ). As above, we obtain information about the nature of the circle associated with k = from both the definition of Apollonian circles and from the equation ( x aλ) y a ( λ ) the definition of Apollonian circles, d ( P, B) d ( P, A) + =. From =. This is the definition of the perpendicular bisector of points A and B. From the equation of the circle, as λ approaches, the center of the circle moves along the x-axis away from the origin, and the radius of the circle approaches. In essence, this corresponds to a circle centered at a point infinitely far from the origin along the x-axis with an infinite radius. When 0 k < <, d ( P, B) d ( P, A) <, which implies that point P is closer to point B than to point A. Indeed, λ is always positive for these values of k. From the equation of the circles, the center of the circle lies to the right of B( a,0) on the x-axis. The radius is ( λ ) a = aλ a, so the distance from the origin to the center of the circle is greater than the radius of the circle, hence, these circles do not intersect the y-axis. When k >, d ( P, B) d ( P, A) >, which implies that point P is closer to point A than to point B. Indeed, λ is always negative for these values of k. From the equation of the circles, the center of the circle lies to the left of A( a,0) on the x-axis. We now know the structure of the red family the Apollonian family of circles. They all have an equation of the form ( x aλ) y a ( λ ) λ + k k =. Depending on the value of the parameter ( k) + = where λ where k 0, we obtain every circle in the family. We now establish a value of a for the coaxal system that is derived from the factors of 0.

15 Coaxal Circles for the Gaussian Factors of 0. Figure has circles colored blue and red to represent the coaxal family to which they belong. Points A and B are the limiting points of the coaxal family every circle in the blue family (provided we can find others) must pass through those two points. If we determine the coordinates of those limiting points, we see that point A has coordinates ( 0,0) and point B has coordinates ( 0,0). Points A and B are located at the square roots of 0 on the real axis! x A = -3.6 x B = 3.6 x A 6= 0.00 x B = A - B Figure. The limiting points are! Using the blue circles to determine the value of a is 0. In Figure, choose a particular pair of factors of 0, say, + iand i. The product of these two factors is 0 with a circle from the blue family that passes through them both. Recalling that each circle in the blue family must have its center in the y-axis, every circle must have an equation of the form ( ) x + y h = r. However, every one of these circles passes through the limiting points (,0) and (,0) A a B a blue circles as ( ) x + y hy = a. or -7. Using the Pythagorean Theorem, we rewrite the equation of the x + y h = h + a, or by expanding and collecting like terms, We know two points on this circle: (, ) and (, ) + = = and ( ) ( ) h h a ± 0. Therefore, + h = 0 + h = a

16 or a = ±. This means that a = 0 or 0 h = 0 + h h =. Do we obtain the same result for another pair of factors, say, + 3i and 3i? As x + y hy = a, so above, they must satisfy the equation or or + ( ) h = h = a and ( ) ( ) a = ±. This means that a = 0 or 0 0 6h = 0 + 6h h = 0 + 6h = a h = 0. Now consider the coaxal circles further. In Figure, if there are other circles in the blue family, they must all be orthogonal to the circles in the red family. Furthermore, based on the discussion of coaxal circles, their centers must lie on the y-axis. Likewise, if there are other circles in the red family, they must be orthogonal to every circle in the blue family. Just as with the blue circles, every circle in the red family has its center on the x-axis. Again, referring back to the coaxal circles discussion, the y-axis must be a member of the red family of circles, and the x-axis must be a member of the blue circles. If we consider the ordinary integer factors of 0 (,,, and 0) all lie on the x-axis, and the axis passes through the limiting points A and B, is seems even more reasonable to think that the x-axis is a member of the blue family of circles. With an understanding of coaxal circles, in particular, where the centers of the circles lie, we construct circles from both families that pass through the Gaussian factors of 0 as in Figure. To see a comparison, the factors are included with and without the circles.

17 Figure a. All The Gaussian Factors of Figure b. All The Gaussian Factors of 0 with the coaxal family of circles -0 - Recall any circle from the blue family intersects any circle from the red family at a 90. Furthermore, circles from different families intersect in two points. Granted, the two circles need not intersect at a lattice point (which is a Gaussian integer), however, we explore two circles from this coaxal system one circle from each family that intersect at Gaussian integers as in Figure 6. In this particular example, the intersection points of the circles are i and + i. The product of these two intersection points is 0. Each pair of factors of 0 is the pair of intersection points of a circle in the blue family and a circle from the red family.

18 8 6 +i i - Figure 6. The intersection points are the factor pairs of 0! Now consider two other circles whose intersections points are not Gaussian integers. In Figure 7, point K is approximately approximately 3.69.i. Their product is i and point L is -. + i.3 K x K = -. y K =.3 x L = y L = -. x y x K L-y y K x K L +x L L = 0.00 y K = L i -. - Figure 7. The product of the intersections of any two circles is 0!

19 Other Oddities of the Gaussian Factors. There are some other strange behaviors referred to earlier. Recall that ( i)( i) 3 = It appears that 3 is not a prime number in the Gaussian integers! 3 has no factors other than and 3 in the ordinary integers; however, we can find factors in the Gaussian integers. 3 has sixteen Gaussian factors seen in Figure 8. Though having relatively few factors, we can still construct a coaxal system of circles with a circle from each family passing through two factors of 3. Indeed, if 3 can be factored as αβ, then there is a circle from each family that passes through α and β. The limiting points of this coaxal system are. ± 3 Factors of The Limiting Points x A = -3.6 x B = 3.6 A B Figure 8. The Gaussian Factors of 3! 3 is not prime! Gaussian Factors of 0. To provide other examples of Gaussian factor patterns, consider the Gaussian factors of 0 in Figure 9. When graphed in the plane, the factors look strikingly similar to the factors of 0. Though not in the figure, the limiting points are indeed. ± 0

20 0 Factors of Figure 9a. The Gaussian Factors of Figure 9b The Gaussian Factors of 0 with the Coaxal Circles - -0

21 References. Brannan, D.A., Esplen, M.F., Gray, J.J. (999). Geometry. Cambridge University Press. Coxeter, H.S.M., Greitzer, S.L. (967). Geometry revisited, New Mathematical Library, vol. 9. The Mathematical Association of America. Henle, M. (00) Modern Geometries: non-euclidean, projective and discrete, nd Edition. Prentice Hall. Needham, T. (997). Visual complex analysis. Oxford University Press.

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