COMMENT REDUCE INTERACTIVE LESSON NUMBER 1 David R. Stoutemyer University of Hawaii COMMENT This is lesson 1 of 7 interactive lessons about the REDUCE system for computer symbolic mathematics. These lessons presume an acquaintance with elementary calculus, together with a previous exposure to some computer programming language. These lessons have been designed for use on a DEC system 10 or 20. Apart from changes to the prompt and interrupt characters however they should work just as well with any REDUCE implementation. In REDUCE, any sequence of characters from the word "COMMENT" through the next semicolon or dollar-sign statement separator is an explanatory remark ignored by the system. In general, either separator signals the end of a statement, with the dollar sign suppressing any output that might otherwise automatically be produced by the statement. The typing of a carriage return initiates the immediate sequential execution of all statements which have been terminated on that line. When REDUCE is ready for more input, it will prompt you with an asterisk at the left margin. To terminate the lesson and return to the operating system, type an interrupt character (DEC: control-C ) at any time. Expressions can be formed using "**", "*", "/", "+", and "-" to indicate exponentiation, multiplication, division, addition, and subtraction or negation respectively. Assignments to variables can be done using the operator ":=". For example:; R2D2 := (987654321/15)**3; COMMENT The immediately preceding line, without a semicolon, is the computed output generated by the line with a semicolon which precedes it. Note that exact indefinite-precision rational arithmetic was used, in contrast to the limited-precision arithmetic of traditional programming languages. We can use the name R2D2 to represent its value in subsequent expressions such as; R2D2 := -R2D2/25 + 3*(13-5); COMMENT Now I will give you an opportunity to try some analogous computations. To do so, type the letter N followed by a carriage return in response to our question "CONT?" (You could type Y if you wish to relinquish this opportunity, but I strongly recommend reinforced learning through active participation.) After trying an example or two, type the command "CONT" terminated by a semicolon and carriage return when you wish to proceed with the rest of the lesson. To avoid interference with our examples, please don't assign anything to any variable names beginning with the letters E through I. To avoid lengthy delays, I recommend keeping all of your examples approximately as trivial as ours, saving your more ambitious experiments until after the lesson. If you happen to initiate a calculation requiring an undue amount of time to evaluate or to print, you can abort that computation with an interrupt to get back to the operating system. Restart REDUCE, followed by the statement "IN LESS1", followed by a semicolon and return, to restart the lesson at the beginning; PAUSE; COMMENT Now watch this example illustrating some more dramatic differences from traditional scientific programming systems:; E1 := 2*G + 3*G + H**3/H; COMMENT Note how we are allowed to use variables to which we have assigned no values! Note too how similar terms and similar factors are combined automatically. REDUCE also automatically expands products and powers of sums, together with placing expressions over common denominators, as illustrated by the examples:; E2 := E1*(F+G); E2 := E1**2; E1+1/E1; COMMENT Our last example also illustrates that there is no need to assign an expression if we do not plan to use its value later. Try some similar examples:; PAUSE; COMMENT It is not always desirable to expand expressions over a common denominator, and we can use the OFF statement to turn off either or both computational switches which control these transformations. The switch named EXP controls EXPansion, and the switch named MCD controls the Making of Common Denominators; OFF EXP, MCD; E2 := E1**2 $ E2 := E2*(F+G) + 1/E1; COMMENT To turn these switches back on, we type:; ON EXP, MCD; COMMENT Try a few relevant examples with these switches turned off individually and jointly; PAUSE; ON EXP; % Just in case you turned it off. COMMENT Now consider the example:; E2 := (2*(F*H)**2 - F**2*G*H - (F*G)**2 - F*H**3 + F*H*G**2 - H**4 + G*H**3)/(F**2*H - F**2*G - F*H**2 + 2*F*G*H - F*G**2 - G*H**2 + G**2*H); COMMENT It is not obvious, but the numerator and denominator of this expression share a nontrivial common divisor which can be canceled. To make REDUCE automatically cancel greatest common divisors, we turn on the computational switch named GCD:; ON GCD; E2; COMMENT The switch is not on by default because 1. It can consume a lot of time. 2. Often we know in advance the few places where a nontrivial GCD can occur in our problem. 3. Even without GCD cancellation, expansion and common denomin- ators guarantee that any rational expression which is equiv- alent to zero simplifies to zero. 4. When the denominator is the greatest common divisor, such as for (X**2 - 2*X + 1)/(X-1), REDUCE cancels the greatest common divisor even when GCD is OFF. 5. GCD cancellation sometimes makes expressions more complicated, such as with (F**10 - G**10)/(F**2 + F*G -2*G**2). Try the examples mentioned in this comment, together with one or two other relevant ones; PAUSE; COMMENT Exact rational arithmetic can consume an alarming amount of computer time when the constituent integers have quite large magnitudes, and the results become awkward to interpret qualitatively. When this is the case and somewhat inexact numerical coefficients are acceptable, we can have the arithmetic done floating point by turning on the computational switch ROUNDED. With this switch on, any non-integer rational numbers are approximated by floating-point numbers, and the result of any arithmetic operation is floating-point when any of its operands is floating point. For example:; ON ROUNDED; E1:= (12.3456789E3 *F + 3*G)**2 + 1/2; COMMENT With ROUNDED off, any floating-point constants are automatically approximated by rational numbers:; OFF ROUNDED; E1 := 12.35*G; PAUSE; COMMENT A number of elementary functions, such as SIN, COS and LOG, are built into REDUCE. Moreover, the letter E represents the base of the natural logarithms, so the exponentiation operator enables us to represent the exponential function as well as fractional powers. For example:; E1:= SIN(-F*G) + LOG(E) + (3*G**2*COS(-1))**(1/2); COMMENT What automatic simplifications can you identify in this example? Note that most REDUCE implementations do not approximate the values of these functions for non-trivial numerical arguments, and exact computations are generally impossible for such cases. Experimentally determine some other built-in simplifications for these functions; PAUSE; COMMENT Later you will learn how to introduce additional simplifications and additional functions, including numerical approximations for examples such as COS(1). Differentiation is also built-into REDUCE. For example, to differentiate E1 with respect to F; E2 := DF(E1,F); COMMENT To compute the second derivative of E2 with respect to G, we can type either DF(E2,G,2) or DF(E1,F,1,G,2) or DF(E1,F,G,2) or DF(E1,G,2,F,1) or; DF(E1,G,2,F); COMMENT Surely you can't resist trying a few derivatives of your own! (Careful, High-order derivatives can be alarmingly complicated); PAUSE; COMMENT REDUCE uses the name I to represent (-1)**(1/2), incorporating some simplification rules such as replacing I**2 by -1. Here is an opportunity to experimentally determine other simplifications such as for I**3, 1/I**23, and (I**2-1)/(I-1); PAUSE; COMMENT Clearly it is inadvisable to use E or I as a variable. T is also inadvisable for reasons that will become clear later. The value of a variable is said to be "bound" to the variable. Any variable to which we have assigned a value is called a bound variable, and any variable to which we have not assigned a value is called an indeterminate. Occasionally it is desirable to make a bound variable into an indeterminate, and this can be done using the CLEAR command. For example:; CLEAR R2D2, E1, E2; E2; COMMENT If you suspect that a degenerate assignment, such as E1:=E1, would suffice to clear a bound variable, try it on one of your own bound variables:; PAUSE; COMMENT REDUCE also supports matrix algebra, as illustrated by the following sequence:; MATRIX E1(4,1), F, H; COMMENT This declaration establishes E1 as a matrix with 4 rows and 1 column, while establishing F and H as matrices of unspecified size. To establish element values (and sizes if not already established in the MATRIX declaration), we can use the MAT function, as illustrated by the following example:; H := MAT((LOG(G), G+3), (G, 5/7)); COMMENT Only after establishing the size and establishing the element values of a declared matrix by executing a matrix assignment can we refer to an individual element or to the matrix as a whole. For example to increase the last element of H by 1 then form twice the transpose of H, we can type; H(2,2) := H(2,2) + 1; 2*TP(H); COMMENT To compute the determinant of H:; DET(H); COMMENT To compute the trace of H:; TRACE(H); COMMENT To compute the inverse of H, we can type H**(-1) or 1/H. To compute the solution to the equation H*F = MAT((G),(2)), we can left-multiply the right-hand side by the inverse of H:; F := 1/H*MAT((G),(2)); COMMENT Notes: 1. MAT((G),(2))/H would denote right-multiplication by the inverse, which is not what we want. 2. Solutions for a set of right-hand-side vectors are most efficiently computed simultaneously by collecting the right- hand sides together as the columns of a single multiple-column matrix. 3. Subexpressions of the form 1/H*... or H**(-1)*... are computed more efficiently than if the inverse is computed separately in a previous statement, so separate computation of the inverse is advisable only if several solutions are desired and if they cannot be computed simultaneously. 4. MAT must have parentheses around each row of elements even if there is only one row or only one element per row. 5. References to individual matrix elements must have exactly two subscripts, even if the matrix has only one row or one column. Congratulations on completing lesson 1! I urge you to try a sequence of more ambitious examples for the various features that have been introduced, in order to gain some familiarity with the relationship between problem size and computing time for various operations. (In most implementations, the command "ON TIME" causes computing time to be printed.) I also urge you to bring to the next lesson appropriate examples from textbooks, articles, or elsewhere, in order to experience the decisive learning reinforcement afforded by meaningful personal examples that are not arbitrarily contrived. To avoid the possibility of interference from assignments and declar- ations in lesson 1, it is wise to execute lesson 2 in a fresh REDUCE job, when you are ready. ;END;