# Known errors in draft13.pdf

creditpagecorrection
VDA 9 In line 4, “improves” should be “improves”.
VDA 22 In the section “Finding methods for objects,” the example defines the expression rats using the ** operator. Unfortunately, the ** operator hasn’t actually been defined yet; that occurs on p. 27. It seems better to define it as

rats = 1/x + 1/2

which behaves the same way and is simpler to boot. Some relevant wording has to be changed to reflect the new expression.
JP 22 Building on VDA’s observation of the example, JP discovered that some versions of Sage actually do simplify these expressions when using the simplify() command; for instance, Sage 6.7 does. Others do not, such as Sage 8.1. So we add a footnote that states,

It does actually work in some versions of Sage, such as Sage 6.7, but not in others, such as 8.1. If it simplifies in your version, just pretend that it doesn't for the sake of the argument, and follow along, since that isn't the point anyway.”

VDA 26Unintentional repetition of “as well”.
VDA 31Unintentional reptition of “this”.
VDA 32 By ”raised x” we mean the multiplication symbol (×, $\times$), which is sometimes raised above the baseline of the text (perhaps you will see that here on the webpage, where to the writer it seems raised ever-so-little). Ironically, and somewhat confusingly, the typeface we chose for the text does not raise the symbol above the baseline. So it’s a good idea to strike the word “raised” to avoid confusion.
VDA 34 The text refers to “three ways” to substitute into an expression, but since it doesn’t alert the reader to when it starts describing the second way, encountering the explicitly named third way can confuse the reader. The “second way” is the syntax f(x=2) or f({x:2}), where you need not use the .subs() method.
VDA 39 “1042th” should be “1042nd”.
VDA 40 Is q positive, negative or zero?” should have p instead of q.
VDA 44 The phrasing, “That last one is not actually in the complex ring $\mathbb C$” might not be clear, since in fact $1+i\in\mathbb C$. What is meant is that Sage doesn’t seem to recognize that $1+i\in\mathbb C$.
VDA 48 In VDA’s memorable explanation, “The Bonus at the end of the exercises is, wrongly, numbered, or in any case is wrongly numbered”
VDA 50 The multiple choice questions refer to ind, but the text does not explain what that is. It now adds an example, $\lim_{x\rightarrow0}\sin\left(\frac{1}{x}\right)\ ,$ where Sage returns that as an answer, and explains that
In some cases, the limit doesn't exist because the function doesn't approach any particular value. When this happens, but the function remains finite, Sage will reply with ind, which is short for, “the limit is indefinite, but bounded [that is, not infinite].” We see that in the [new] example. If however the function waggles between infinities, Sage will reply with und.
VDA 52 “Both commands” refers to point() and line(), not to arrow().
VDA 53 The default for zorder actually depends on the object. For some it is 1, for others it is 3.
JK 55 The sentence, “Why this option called zorder?” should be, “Why is this option called zorder?”
VDA 58 “We consider three more objects” — yes, but remove the reference to text(), which is discussed in the subsequent section anyway.
VDA 65 The explanation of the parametric_plot() command will probably make a lot more sense if we replace tvar by t.
VDA 68 The suggestion to “[p]lay around with different values of tmin and tmax to see why $[0,2\pi)$ gives us a complete wave” is, strictly speaking, correct, but it may be more instructive “to determine the smallest interval $[a,b]$ that gives us a complete wave.”
VDA 71 While the discussion on the DomainError is, strictly speaking, mostly correct, it is a bit out of place, and besides, it seems… unnatural to plot $\sqrt{6-x^2}$ on the interval $(-3,3)$ instead of the interval $(-\sqrt6,\sqrt6)$. It is certainly erroneous to plot it by asking Sage to plot sqrt(5-x^2). (This last bit is why the discussion is only mostly correct.)
VDA 72 The line before the last equation has an extra “transforms.”
BB 140 The statement “This touches on an topic called…” has a rather obvious error.
JD 154 Multiple Choice Exercise 5 ought to ask something different from Exercise 4. Try, “The result of .roots() when applied to an equation in one variable is:”.
JS p. 264 The lab “An important difference quotient” neglects to mention that $f(x)=x^n$, which is sort of important if the student is to complete the latter half of the lab.
JD Multiple pages throughout the Encyclopædia Laboratorica The text refers to “text cell” when “HTML cell” is more appropriate (at least in CoCalc)
JK 285 The lab “Properties of finite rings” refers to a lecture that corresponds to the personal notes of one of the authors. Even worse, after several years of revision it corresponds to the wrong lecture. These days it should refer to the section “Mathematical structures in Sage” in the chapter titled, “Basic Computations.”
WM 286 The lab “The Chinese Remainder Clock” instructs students to count “clockwise from the top” but the remainder of dividing the hour by 3 is curiously positioned counterclockwise — and is, curiously enough, the only one so misplaced. The correct image should be:

#### Legend for credits:

VDA
Valerio De Angelis, Xavier University, who probably deserves co-author credit by now
BB
Baylee Bourque
JD