Let [Formula: see text], where [Formula: see text] are integers and [Formula: see text] be a digit set. Then the pair [Formula: see text] generates a fractal set [Formula: see text] satisfying [Formula: see text] which is a unit square. However, if we remove one digit from [Formula: see text], then the structure of [Formula: see text] will become very interesting. A well-known example is the Sierpinski carpet. In this paper, we study the resulting self-affine sets of moving a digit in [Formula: see text] to a different place. That is, we consider a digit set [Formula: see text], where [Formula: see text]. We give a complete characterization for the connectedness of self-affine carpet [Formula: see text] in terms of the domains of [Formula: see text] and [Formula: see text].

In the paper, we focus on the connectedness of planar self-affine sets T (A, D) generated by an integer expanding matrix A with | det(A)| = 3 and a collinear digit set D = {0, 1, b}v, where b > 1 and v ∈ R 2 such that {v, Av} is linearly independent. We discuss the domain of the digit b to determine the connectedness of T (A, D). Especially, a complete characterization is obtained when we restrict b to be an integer. Some results on the general case of | det(A)| > 3 are obtained as well.

Let A be an expanding integer matrix with characteristic polynomial f (x) = x 2 + px + q, and let D = {0, 1, . . . , |q| − 2, |q| + m}v be a collinear digit set where m 0, v ∈ Z 2 . It is well known that there exists a unique self-affine fractal T satisfying AT = T + D. In this paper, we give a complete characterization on the connected T . That generalizes the previous result of |q| = 3.

The theory of quadratic equations (with real coefficients) is an important topic in the secondary school mathematics curriculum. Usually students are taught to solve a quadratic equation ax2 + bx + c = 0 (a ≠ 0) algebraically (by factorisation, completing the square, quadratic formula), graphically (by plotting the graph of the quadratic polynomial y = ax2 + bx + c to find the x-intercepts, if any), and numerically (by the bisection method or Newton-Raphson method). Less well-known is that we can indeed solve a quadratic equation geometrically (by geometric construction tools such as a ruler and compasses, R&C for short). In this article we describe this approach. A more comprehensive discussion on geometric approaches to quadratic equations can be found in [1]. We have also gained much insight from [2] to develop our methods. The tool we use is a set square rather than the more common R&C. But the methods to be presented here can also be carried out with R&C. We choose a set square because it is more convenient (one tool is used instead of two).

We can obtain a two-fold symmetric figure by folding a square sheet of paper in the middle and then cutting along some curves drawn on the paper. By making two perpendicular folds through the centre of the paper and then cutting, we can obtain a four-fold symmetric figure. We can also get an eight-fold symmetric figure by making a fold bisecting an angle made by the two perpendicular folds before cutting. But it is not possible to obtain a three-fold, five-fold or six-fold symmetric figure in this way; we need to make more folds before cutting. Making a three-fold (respectively five-fold and six-fold) figure involves the division of the angle at the centre (360°) of a square sheet of a paper into six (respectively ten and twelve) equal parts. In other words, we need to construct the angles 60°, 36° and 30°. But these angles cannot be obtained by repeated bisections of 180° by simple folding as in the making of two-fold, four-fold and eight-fold figures. In [1], we see that each of the constructions of 60° and 30° applies the fact that sin 30° = ½ and takes only a few simple folding steps. The construction of 36° is more tedious (see, for example, [2] and [3]) as sin 36° is not a simple fraction but an irrational number. In this Article, we show how to make, by paper-folding and cutting a regular pentagon, a five-pointed star and create any five-fold figure as we want. The construction obtained by dividing the angle at the centre of a square paper into ten equal parts is called a pentagon base. We gained much insight from [2] and [3] when developing the method for making the pentagon base to be presented below.