The convention of rows first and columns secondmust be followed. Vote. Dividing two (or more) matrices is more involved than A matrix is an array of elements (usually numbers) that has a set number of rows and columns. You can have a look at our matrix multiplication instructions to refresh your memory. of a matrix or to solve a system of linear equations. Continuing in this way, we keep choosing vectors until we eventually do have a linearly independent spanning set: say \(V = \text{Span}\{v_1,v_2,\ldots,v_m,\ldots,v_{m+k}\}\). As we've mentioned at the end of the previous section, it may happen that we don't need all of the matrix' columns to find the column space. Oh, how fortunate that we have the column space calculator for just this task! Pick the 2nd element in the 2nd column and do the same operations up to the end (pivots may be shifted sometimes). matrices A and B must have the same size. In general, if we have a matrix with $ m $ rows and $ n $ columns, we name it $ m \times n $, or rows x columns. becomes \(a_{ji}\) in \(A^T\). We pronounce it as a 2 by 2 matrix. (Unless you'd already seen the movie by that time, which we don't recommend at that age.). The best answers are voted up and rise to the top, Not the answer you're looking for? A^2 & = A \times A = \begin{pmatrix}1 &2 \\3 &4 Recently I was told this is not true, and the dimension of this vector space would be $\Bbb R^n$. &b_{2,4} \\ \color{blue}b_{3,1} &b_{3,2} &b_{3,3} &b_{3,4} \\ a bug ? These are the ones that form the basis for the column space. A basis, if you didn't already know, is a set of linearly independent vectors that span some vector space, say $W$, that is a subset of $V$. Write to dCode! In this case, the array has three rows, which translates to the columns having three elements. To calculate a rank of a matrix you need to do the following steps. In fact, we can also define the row space of a matrix: we simply repeat all of the above, but exchange column for row everywhere. First transposed the matrix: M T = ( 1 2 0 1 3 1 1 6 1) Now we use Gauss and get zero lines. "Alright, I get the idea, but how do I find the basis for the column space?" \[V=\left\{\left(\begin{array}{c}x\\y\\z\end{array}\right)|x+2y=z\right\}.\nonumber\], Find a basis for \(V\). whether two matrices can be multiplied, and second, the elements in matrix \(C\). Use Wolfram|Alpha for viewing step-by-step methods and computing eigenvalues, eigenvectors, diagonalization and many other properties of square and non-square matrices. Once you've done that, refresh this page to start using Wolfram|Alpha. When referring to a specific value in a matrix, called an element, a variable with two subscripts is often used to denote each element based on its position in the matrix. You can remember the naming of a matrix using a quick mnemonic. \\\end{pmatrix}^2 \\ & = Since \(A\) is a square matrix, it has a pivot in every row if and only if it has a pivot in every column. the matrix equivalent of the number "1." a 4 4 being reduced to a series of scalars multiplied by 3 3 matrices, where each subsequent pair of scalar reduced matrix has alternating positive and negative signs (i.e. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. In other words, if you already know that \(\dim V = m\text{,}\) and if you have a set of \(m\) vectors \(\mathcal{B}= \{v_1,v_2,\ldots,v_m\}\) in \(V\text{,}\) then you only have to check one of: in order for \(\mathcal{B}\) to be a basis of \(V\). What is an eigenspace of an eigen value of a matrix? Believe it or not, the column space has little to do with the distance between columns supporting a building. Indeed, a matrix and its reduced row echelon form generally have different column spaces. Feedback and suggestions are welcome so that dCode offers the best 'Eigenspaces of a Matrix' tool for free! Thedimension of a matrix is the number of rows and the number of columns of a matrix, in that order. \times Your dream has finally come true - you've bought yourself a drone! For these matrices we are going to subtract the Accessibility StatementFor more information contact us atinfo@libretexts.org. such as . You can use our adjoint of a 3x3 matrix calculator for taking the inverse of the matrix with order 3x3 or upto 6x6. The dot product involves multiplying the corresponding elements in the row of the first matrix, by that of the columns of the second matrix, and summing up the result, resulting in a single value. \end{align}\); \(\begin{align} B & = \begin{pmatrix} \color{red}b_{1,1} Phew, that was a lot of time spent on theory, wouldn't you say? This is a result of the rank + nullity theorem --> e.g. I am drawing on Axler. respectively, the matrices below are a \(2 2, 3 3,\) and For example, the first matrix shown below is a 2 2 matrix; the second one is a 1 4 matrix; and the third one is a 3 3 matrix. Dimension also changes to the opposite. Recall that the dimension of a matrix is the number of rows and the number of columns a matrix has,in that order. In order to show that \(\mathcal{B}\) is a basis for \(V\text{,}\) we must prove that \(V = \text{Span}\{v_1,v_2,\ldots,v_m\}.\) If not, then there exists some vector \(v_{m+1}\) in \(V\) that is not contained in \(\text{Span}\{v_1,v_2,\ldots,v_m\}.\) By the increasing span criterion Theorem 2.5.2 in Section 2.5, the set \(\{v_1,v_2,\ldots,v_m,v_{m+1}\}\) is also linearly independent. As such, they will be elements of Euclidean space, and the column space of a matrix will be the subspace spanned by these vectors. The addition and the subtraction of the matrices are carried out term by term. Enter your matrix in the cells below "A" or "B". equation for doing so is provided below, but will not be \begin{bmatrix} v_1 \\ v_2 \end{bmatrix} = \begin{bmatrix} 0 \\ 0 \end{bmatrix} $ which has for solution $ v_1 = -v_2 $. An n m matrix is an array of numbers with n rows and m columns. Your vectors have $3$ coordinates/components. \); \( \begin{pmatrix}1 &0 &0 &0 \\ 0 &1 &0 &0 \\ 0 &0 &1 &0 Given: As with exponents in other mathematical contexts, A3, would equal A A A, A4 would equal A A A A, and so on. The matrix product is designed for representing the composition of linear maps that are represented by matrices. Interactive Linear Algebra (Margalit and Rabinoff), { "2.01:_Vectors" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.02:_Vector_Equations_and_Spans" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.03:_Matrix_Equations" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.04:_Solution_Sets" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.05:_Linear_Independence" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.06:_Subspaces" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.07:_Basis_and_Dimension" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.08:_The_Rank_Theorem" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "2.8:_Bases_as_Coordinate_Systems" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { "00:_Front_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "01:_Systems_of_Linear_Equations-_Algebra" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02:_Systems_of_Linear_Equations-_Geometry" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03:_Linear_Transformations_and_Matrix_Algebra" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04:_Determinants" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05:_Eigenvalues_and_Eigenvectors" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06:_Orthogonality" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07:_Appendix" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "zz:_Back_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic", "license:gnufdl", "authorname:margalitrabinoff", "licenseversion:13", "source@https://textbooks.math.gatech.edu/ila" ], https://math.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fmath.libretexts.org%2FBookshelves%2FLinear_Algebra%2FInteractive_Linear_Algebra_(Margalit_and_Rabinoff)%2F02%253A_Systems_of_Linear_Equations-_Geometry%2F2.07%253A_Basis_and_Dimension, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), \(\usepackage{macros} \newcommand{\lt}{<} \newcommand{\gt}{>} \newcommand{\amp}{&} \), Example \(\PageIndex{1}\): A basis of \(\mathbb{R}^2 \), Example \(\PageIndex{2}\): All bases of \(\mathbb{R}^2 \), Example \(\PageIndex{3}\): The standard basis of \(\mathbb{R}^n \), Example \(\PageIndex{6}\): A basis of a span, Example \(\PageIndex{7}\): Another basis of the same span, Example \(\PageIndex{8}\): A basis of a subspace, Example \(\PageIndex{9}\): Two noncollinear vectors form a basis of a plane, Example \(\PageIndex{10}\): Finding a basis by inspection, source@https://textbooks.math.gatech.edu/ila. \\\end{pmatrix} \end{align}$$ $$\begin{align} A^T & = It only takes a minute to sign up. For eigenspace,eigen,space,matrix,eigenvalue,value,eigenvector,vector, What is an eigenspace of an eigen value of a matrix? column of \(C\) is: $$\begin{align} C_{11} & = (1\times7) + (2\times11) + (3\times15) = 74\end{align}$$$$ Seriously. Matrix Determinant Calculator - Symbolab Hence any two noncollinear vectors form a basis of \(\mathbb{R}^2 \). This shows that the plane \(\mathbb{R}^2 \) has dimension 2. As with the example above with 3 3 matrices, you may notice a pattern that essentially allows you to "reduce" the given matrix into a scalar multiplied by the determinant of a matrix of reduced dimensions, i.e. For large matrices, the determinant can be calculated using a method called expansion by minors. So why do we need the column space calculator? The pivot columns of a matrix \(A\) form a basis for \(\text{Col}(A)\). \begin{pmatrix}1 &3 \\2 &4 \\\end{pmatrix} \end{align}$$, $$\begin{align} B & = \begin{pmatrix}2 &4 &6 &8 \\ 10 &12 Kernel of a Matrix Calculator - Math24.pro \\\end{pmatrix} \end{align}$$ $$\begin{align} C^T & = \\\end{pmatrix} \end{align}\); \(\begin{align} s & = 3 Both the Let's take a look at our tool. Exponents for matrices function in the same way as they normally do in math, except that matrix multiplication rules also apply, so only square matrices (matrices with an equal number of rows and columns) can be raised to a power. To raise a matrix to the power, the same rules apply as with matrix For example, you can Just open up the advanced mode and choose "Yes" under "Show the reduced matrix?". So how do we add 2 matrices? must be the same for both matrices. It is used in linear algebra, calculus, and other mathematical contexts. This is the idea behind the notion of a basis. A matrix is an array of elements (usually numbers) that has a set number of rows and columns. 1 + 4 = 5\end{align}$$ $$\begin{align} C_{21} = A_{21} + The determinant of \(A\) using the Leibniz formula is: $$\begin{align} |A| & = \begin{vmatrix}a &b \\c &d \end{align} \), We will calculate \(B^{-1}\) by using the steps described in the other second of this app, \(\begin{align} {B}^{-1} & = \begin{pmatrix}\frac{1}{30} &\frac{11}{30} &\frac{-1}{30} \\\frac{-7}{15} &\frac{-2}{15} &\frac{2}{3} \\\frac{8}{15} &\frac{-2}{15} &\frac{-1}{3} Why xargs does not process the last argument? Below are descriptions of the matrix operations that this calculator can perform. Let \(v_1,v_2\) be vectors in \(\mathbb{R}^2 \text{,}\) and let \(A\) be the matrix with columns \(v_1,v_2\). With what we've seen above, this means that out of all the vectors at our disposal, we throw away all which we don't need so that we end up with a linearly independent set. \begin{align} This means the matrix must have an equal amount of The previous Example \(\PageIndex{3}\)implies that any basis for \(\mathbb{R}^n \) has \(n\) vectors in it. \\\end{pmatrix}\end{align}$$. We need to input our three vectors as columns of the matrix. Assuming that the matrix name is B B, the matrix dimensions are written as Bmn B m n. The number of rows is 2 2. m = 2 m = 2 The number of columns is 3 3. n = 3 n = 3 The individual entries in any matrix are known as. Since A is 2 3 and B is 3 4, C will be a 2 4 matrix. An attempt to understand the dimension formula. The dimension of \(\text{Col}(A)\) is the number of pivots of \(A\). scalar, we can multiply the determinant of the \(2 2\) Refer to the matrix multiplication section, if necessary, for a refresher on how to multiply matrices. \begin{align} C_{12} & = (1\times8) + (2\times12) + (3\times16) = 80\end{align}$$$$ After all, we're here for the column space of a matrix, and the column space we will see! \\\end{pmatrix} \end{align} $$. Desmos | Matrix Calculator The Leibniz formula and the To have something to hold on to, recall the matrix from the above section: In a more concise notation, we can write them as (3,0,1)(3, 0, 1)(3,0,1) and (1,2,1)(-1, 2, -1)(1,2,1). We put the numbers in that order with a $ \times $ sign in between them. Which results in the following matrix \(C\) : $$\begin{align} C & = \begin{pmatrix}2 & -3 \\11 &12 \\4 & 6 of matrix \(C\), and so on, as shown in the example below: \(\begin{align} A & = \begin{pmatrix}1 &2 &3 \\4 &5 &6 Below is an example So we will add \(a_{1,1}\) with \(b_{1,1}\) ; \(a_{1,2}\) with \(b_{1,2}\) , etc. matrix kernel - Wolfram|Alpha Let \(V\) be a subspace of dimension \(m\). We write two linear combinations of the four given spanning vectors, chosen at random: \[w_1=\left(\begin{array}{c}1\\-2\\2\end{array}\right)+\left(\begin{array}{c}2\\-3\\4\end{array}\right)=\left(\begin{array}{c}3\\-5\\6\end{array}\right)\quad w_2=-\left(\begin{array}{c}2\\-3\\4\end{array}\right)+\frac{1}{2}\left(\begin{array}{c}0\\4\\0\end{array}\right)=\left(\begin{array}{c}-2\\5\\-4\end{array}\right).\nonumber\]. You close your eyes, flip a coin, and choose three vectors at random: (1,3,2)(1, 3, -2)(1,3,2), (4,7,1)(4, 7, 1)(4,7,1), and (3,1,12)(3, -1, 12)(3,1,12). At first glance, it looks like just a number inside a parenthesis. This is read aloud, "two by three." Note: One way to remember that R ows come first and C olumns come second is by thinking of RC Cola . \(V = \text{Span}\{v_1,v_2,\ldots,v_m\}\text{,}\) and. There are a number of methods and formulas for calculating Link. Connect and share knowledge within a single location that is structured and easy to search. There are two ways for matrix multiplication: scalar multiplication and matrix with matrix multiplication: Scalar multiplication means we will multiply a single matrix with a scalar value. D=-(bi-ch); E=ai-cg; F=-(ah-bg) In essence, linear dependence means that you can construct (at least) one of the vectors from the others. Set the matrix. In order to find a basis for a given subspace, it is usually best to rewrite the subspace as a column space or a null space first: see this note in Section 2.6, Note 2.6.3. \end{pmatrix} \end{align}$$, $$\begin{align} C & = \begin{pmatrix}2 &4 \\6 &8 \\10 &12 row 1 of \(A\) and column 1 of \(B\): $$ a_{11} \times b_{11} + a_{12} \times b_{21} + a_{13} The dimension of Col(A) is the number of pivots of A. matrix. This algorithm tries to eliminate (i.e., make 000) as many entries of the matrix as possible using elementary row operations. \times The Column Space Calculator will find a basis for the column space of a matrix for you, and show all steps in the process along the way. \begin{align} C_{13} & = (1\times9) + (2\times13) + (3\times17) = 86\end{align}$$$$ Well, this can be a matrix as well. The elements of a matrix X are noted as x i, j , where x i represents the row number and x j represents the column number. Why did DOS-based Windows require HIMEM.SYS to boot? \(A A\) in this case is not possible to calculate. Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. The binomial coefficient calculator, commonly referred to as "n choose k", computes the number of combinations for your everyday needs. The null space always contains a zero vector, but other vectors can also exist. 2.7: Basis and Dimension - Mathematics LibreTexts For example, given two matrices A and B, where A is a m x p matrix and B is a p x n matrix, you can multiply them together to get a new m x n matrix C, where each element of C is the dot product of a row in A and a column in B. Matrix Multiply, Power Calculator - Symbolab Matrix rank is calculated by reducing matrix to a row echelon form using elementary row operations. Each row must begin with a new line. You need to enable it. \\ 0 &0 &0 &1 \end{pmatrix} \cdots \), $$ \begin{pmatrix}1 &0 &0 &\cdots &0 \\ 0 &1 &0 &\cdots &0 For example, the number 1 multiplied by any number n equals n. The same is true of an identity matrix multiplied by a matrix of the same size: A I = A. Take the first line and add it to the third: M T = ( 1 2 0 0 5 1 1 6 1) Take the first line and add it to the third: M T = ( 1 2 0 0 5 1 0 4 1) Thus, we have found the dimension of this matrix. It gives you an easy way to calculate the given values of the Quaternion equation with different formulas of sum, difference, product, magnitude, conjugate, and matrix representation. For math, science, nutrition, history . This gives an array in its so-called reduced row echelon form: The name may sound daunting, but we promise is nothing too hard. The dimension of a vector space who's basis is composed of $2\times2$ matrices is indeed four, because you need 4 numbers to describe the vector space. For a vector space whose basis elements are themselves matrices, the dimension will be less or equal to the number of elements in the matrix, this $\dim[M_2(\mathbb{R})]=4$. used: $$\begin{align} A^{-1} & = \begin{pmatrix}a &b \\c &d Check out 35 similar linear algebra calculators , Example: using the column space calculator. If you're feeling especially brainy, you can even have some complex numbers in there too. Eigenspaces of a Matrix Calculator - Online Eigen Spaces Finder - dCode In other words, if \(\{v_1,v_2,\ldots,v_m\}\) is a basis of a subspace \(V\text{,}\) then no proper subset of \(\{v_1,v_2,\ldots,v_m\}\) will span \(V\text{:}\) it is a minimal spanning set. &h &i \end{pmatrix} \end{align}$$, $$\begin{align} M^{-1} & = \frac{1}{det(M)} \begin{pmatrix}A What we mean by this is that we can obtain all the linear combinations of the vectors by using only a few of the columns. \\\end{pmatrix} \end{align}$$. We see that the first one has cells denoted by a1a_1a1, b1b_1b1, and c1c_1c1. \\\end{pmatrix} The first number is the number of rows and the next number is thenumber of columns. For a matrix $ M $ having for eigenvalues $ \lambda_i $, an eigenspace $ E $ associated with an eigenvalue $ \lambda_i $ is the set (the basis) of eigenvectors $ \vec{v_i} $ which have the same eigenvalue and the zero vector. concepts that won't be discussed here. dimension of R3 = rank(col(A)) + null(A), or 3 = 2 + 1. Quaternion Calculator is a small size and easy-to-use tool for math students. \[V=\left\{\left(\begin{array}{c}x_1\\x_2\\x_3\end{array}\right)|x_1 +x_2=x_3\right\}\nonumber\], by inspection. Rank is equal to the number of "steps" - the quantity of linearly independent equations. To say that \(\{v_1,v_2,\ldots,v_n\}\) spans \(\mathbb{R}^n \) means that \(A\) has a pivot position, To say that \(\{v_1,v_2,\ldots,v_n\}\) is linearly independent means that \(A\) has a pivot position in every. The identity matrix is a square matrix with "1" across its diagonal, and "0" everywhere else. Understand the definition of a basis of a subspace. diagonal, and "0" everywhere else. \\\end{vmatrix} \end{align} = {14 - 23} = -2$$. You can have number or letter as the elements in a matrix based on your need. \begin{align} C_{22} & = (4\times8) + (5\times12) + (6\times16) = 188\end{align}$$$$ It is a $ 3 \times 2 $ matrix. Why use some fancy tool for that? Laplace formula and the Leibniz formula can be represented &\cdots \\ 0 &0 &0 &\cdots &1 \end{pmatrix} $$. the elements from the corresponding rows and columns. Determinant of a 4 4 matrix and higher: The determinant of a 4 4 matrix and higher can be computed in much the same way as that of a 3 3, using the Laplace formula or the Leibniz formula. \(2 4\) matrix. Given: $$\begin{align} |A| & = \begin{vmatrix}1 &2 \\3 &4 \begin{pmatrix}3 & 5 & 7 \\2 & 4 & 6\end{pmatrix}-\begin{pmatrix}1 & 1 & 1 \\1 & 1 & 1\end{pmatrix}, \begin{pmatrix}11 & 3 \\7 & 11\end{pmatrix}\begin{pmatrix}8 & 0 & 1 \\0 & 3 & 5\end{pmatrix}, \det \begin{pmatrix}1 & 2 & 3 \\4 & 5 & 6 \\7 & 8 & 9\end{pmatrix}, angle\:\begin{pmatrix}2&-4&-1\end{pmatrix},\:\begin{pmatrix}0&5&2\end{pmatrix}, projection\:\begin{pmatrix}1&2\end{pmatrix},\:\begin{pmatrix}3&-8\end{pmatrix}, scalar\:projection\:\begin{pmatrix}1&2\end{pmatrix},\:\begin{pmatrix}3&-8\end{pmatrix}. column of \(B\) until all combinations of the two are Linear Algebra Toolkit - Old Dominion University We leave it as an exercise to prove that any two bases have the same number of vectors; one might want to wait until after learning the invertible matrix theorem in Section3.5.