Integer-Digit Functions: An Example of Math- Art Integration ERNESTO ESTRADA ‘‘God made the integers, all else is the work of man.’’ L. Kronecker athematics and the visual arts mutually reinforce Digit Sum with ‘‘Memory’’ one another [10]. On the one hand, many math- A nonnegative integer N can be represented in a given base MMematical objects appear in artistic or decorative b according to the following expression works [5, 6]. In particular, mathematical curves and art have n nÀ1 nÀ2 ; 1 a long-standing connection through the application of N ¼ a1b þ a2b þ a3b þÁÁÁþanÀ1b þ an ð Þ geometric principles [13, 20]. Simple curves such as the where ai are nonnegative integers. We can represent a catenary are ubiquitous, for example in the work of the negative integer in a similar way if we multiply by À1 the Catalan architect Antoni Gaudı´, as well as in ancient [19] right-hand-side part of the previous equation. For instance, and in modern architecture [12]. On the other hand, many the number N ¼ 257, expressed in the base b ¼ 10, is: mathematical objects display artistic appeal per se. The vast 2 list includes, among others: knots [1, 2], mosaics and tiles 257 ¼ 2 Á 10 þ 5 Á 10 þ 7: [3, 21], Fourier series [9], topological tori [15], and fractal A function explored in several contexts [14] is the ‘‘digit curves [4], all of which produce visual patterns of undeni- sum,’’ or ‘‘sum of digits’’ of a given integer, defined by: able beauty. Such connections between mathematics and the arts are explored and celebrated annually by the Xn bclogXb N 1 ÀÁ S ðÞ¼N a ¼ N modbkþ1 À N modbk : ð2Þ Bridges community formed by mathematicians and artists b i bk [8, 18]. i¼1 k¼0 This article introduces a new family of curves in the For instance, for b ¼ 10 the digit sum of 257 is plane defined by functions transforming an integer 2 þ 5 þ 7 ¼ 14. The digit sum forms an integer sequence according to sums of digit-functions. These transforms of registered in the Online Encyclopedia of Integer Sequences an integer N are obtained by multiplying the function value (OEIS) [16, 17] by the sequence A007953. One characteristic fNðÞby the sum of faðÞi , where ai are the digits of N in a of the digit sum function is that it does not ‘‘remember’’ the given base b and f is a standard function such as a integer that gave rise to it. That is, we can find many dif- trigonometric, logarithmic, or exponential one. These ferent integers for which the digit sum is the same. For curves display a few attributes, such as beauty, symmetry, instance, S10ðÞ59 ¼ S10ðÞ77 ¼ S10ðÞ95 ¼ S10ðÞ149 ¼ÁÁÁ¼ and resemblance to natural environments, which make S10ð257 Þ¼ÁÁÁ¼14. them attractive for artistic purposes. Ó 2018 The Author(s). This article is an open access publication, Volume 40, Number 1, 2018 73 https://doi.org/10.1007/s00283-017-9726-x ×104 Integer-Digit Functions 3 Now we extend this idea to other functions. Let fNðÞ be a function of the integer N, for example, sinðÞN . d 2.5 Then we define the sum of digit-functions fNðÞof fNðÞby: d fNðÞ¼ðÞfaðÞþ1 fað2 ÞþÁÁÁþfaðÞn fNðÞ: ð3Þ 2 Here, once again, we are using the previous notation; that is, the nonnegative integers ai are the digits of N in base b. d For instance, sinðÞ¼ 257 ðÞÁsinðÞþ 2 sinðÞþ 5 sinðÞ 7 sinðÞ 257 ˆ x 1.5 À0:2357. For negative integers ÀN , if the function f ðÞÀN exists, we obtain 1 d fðÞÀN ¼ðÞfaðÞ1 þfaðÞ2 þÁÁÁþfaðÞn f ðÞÀN : ð4Þ d As an example, the plot of sinðÞN versus N isgiveninFigure2 0.5 (left panel), which mildly resembles the original sine function. Our main interest here is, however, to make a com- plete transformation of both coordinate axes in terms of 0 0 200 400 600 800 1000 our digit-function. That is, we are interested in the d d d x plotting of fNðÞversus gNðÞ. For instance, when sinðÞN is plotted versus Nb in Figure 2 (right panel), a totally Figure 1. Plot of Nb versus N for integers less than or equal to unexpected pattern emerges. This pattern does not 1000. resemble at all the original function and, if rotated by À90 degrees, it resembles a bush. A simple modification of the digit sum function that In the rest of this article we will be interested in the curves d d ‘‘remembers’’ its origins better can be obtained by multi- we obtain from certain combinations of fNðÞand gNðÞ.To plying the digit sum of N by N [7]. Hereafter we represent represent the curves we use here the following method. For this function by Nb. In the previous example we have a given even number of points n, the curve we want is n 257d ¼ 257 Á 14 ¼ 3598. This function is not different for obtained for the integers N using a parametric form of 2 every integer, thus we cannot say that it remembers its the equation, origins perfectly, but at least it is not as degenerate as the d digit sum. The corresponding integer sequence is given at x ¼ fNðÞ; ð5Þ the OEIS by A117570 [16]. In Figure 1 we illustrate the plot d of Nb versus N for the first 1000 integers. y ¼ gNðÞ: ð6Þ 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 ) ) x x ( ( 0 0 sin sin -0.1 -0.1 -0.2 -0.2 -0.3 -0.3 -0.4 -0.4 -0.5 -0.5 0 200 400 600 800 1000 0 0.5 1 1.5 2 2.5 3 x x ×104 Figure 2. Semideconstruction of the sine function (left) and total deconstruction of the same (right). 74 THE MATHEMATICAL INTELLIGENCER Figure 4. Sine after the integer-digit transform using n ¼ 2000 points (left) and another view of it (right) using colormap ‘‘Jet’’ for artistic impression. To prepare the second image, we Figure 3. Curve defined by Equations (7)–(8) with n ¼ 1000 simply rotate the negative part of the curve to obtain a mirror points. The left panel illustrates the discrete character of the symmetry. On the left-hand side we use red for negative values curve. Note also that negative inputs are distinguishable from of N and blue for positive ones. positive inputs. In the image on the left, red is used for negative values of N and blue for positive ones. The image on the right is obtained by joining the dots with lines to give a more fluid view. In the right panel we used a ‘‘Jet’’ colormap to color the curve. d d If the functions fNðÞand gNðÞare also defined for nega- tive arguments, the corresponding transforms are obtained n for À N . 2 All trigonometric functions are obtained in degrees. For instance, let x ¼ Nb; ð7Þ Figure 5. Involute of the circle after the integer-digit trans- d b d y ¼ sin N À N Á cos N : ð8Þ form using n ¼ 4000 points (left) and a coloring of it using ‘‘Parula’’ color map (right). Then for n ¼ 1000, we obtain the curve illustrated in the left panel of Figure 3. Notice that the curves are discrete because we only evaluate our functions at integer values. However, we can join the points to create a more fluid form; see the right panel of Figure 3, in which we used the x ¼ sindN À Nb Á cosdN ; ð11Þ colormap function available in MatlabÒ.1 y ¼ cosdN þ Nb Á sindN : ð12Þ Artistic Creation Using Integer-Digit Functions and the corresponding curve is illustrated in Figure 5. Now we play with this construction and transform some The analogue of the astroid using the integer-digit well-known curves, such as sine, involute of a circle, transform is given by astroid, and others. Let us start by the simple sine function 3 in which we make the transformation x ¼ cosdN ; ð13Þ b x ¼ N ; ð9Þ 3 y ¼ sindN ; ð14Þ y ¼ sindN : ð10Þ and the corresponding curve for n ¼ 10; 000 points is This function is illustrated in Figure 4 for n ¼ 2000 points. illustrated in Figure 6. The artistic representation of the curve given on the right- The cardioid curve can be transformed using the fol- hand-side of the figure clearly resembles a butterfly and can lowing parametric equations serve as the basis for more artistic inspirations. d We now explore the transformation of both coordinate x ¼ 2cosdN þ cosðÞ 2N ; ð15Þ axes for the involute of a circle. This function is given by d d the following parametric equation y ¼ 2sin N þ 3sinðÞ 2N : ð16Þ 1All images in this article were created in MatlabÒ. Any interested reader can obtain the code from the author via e-mail. Ó 2018 The Author(s). This article is an open access publication, Volume 40, Number 1, 2018 75 Figure 8. Composition of integer-digit transform (left) and another view of the same composition (right). Figure 6. Astroid after the integer-digit transform using n ¼ 10; 000 points (left) and a coloring of it using the colormap In Figure 7 we plot the curve corresponding to the trans- ‘‘Hot’’ and a grey background (right). formed cardioid, which resembles a fish. Although ‘‘beauty is in the eye of the beholder’’ there is no doubt that these plots have certain artistic attributes that overall make them in some way unexpectedly beautiful. In the next section we explore some further possibilities for artistic creation.