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 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 DECsystem 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 dollarsign 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: controlC ) 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 indefiniteprecision rational arithmetic was
 used, in contrast to the limitedprecision arithmetic of traditional
 programming languages.
 We can use the name R2D2 to represent its value in subsequent
 expressions such as;
 R2D2 := R2D2/25 + 3*(135);
 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 flags which control these
 transformations. The flag named EXP controls EXPansion, and the
 flag named MCD controls the Making of Common Denominators;
 OFF EXP, MCD;
 E2 := E1**2 $
 E2 := E2*(F+G) + 1/E1;
 COMMENT To turn these flags back on, we type:;
 ON EXP, MCD;
 COMMENT Try a few relevant examples with these flags turned off
 individually and jointly;
 PAUSE;
 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 cancelled.
 To make REDUCE automatically cancel greatest common divisors, we turn
 on the computational flag named GCD:;
 ON GCD;
 E2;
 COMMENT The flag 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)/(X1), 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).
 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 flag FLOAT. With this flag on,
 any noninteger rational numbers are approximated by floatingpoint
 numbers, and the result of any arithmetic operation is floatingpoint
 when any of its operands is floating point. For example:;
 ON FLOAT, EXP;
 E1:= (12.3456789E3 *F + 3*G)**2 + 1/2;
 COMMENT With FLOAT off, any floatingpoint constants are
 automatically approximated by rational numbers:;
 OFF FLOAT;
 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 nontrivial numerical arguments, and exact
 computations are generally impossible for such cases.
 Experimentally determine some other builtin 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 builtinto 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, Highorder 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**21)/(I1);
 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
 leftmultiply the righthand side by the inverse of H:;
 F := 1/H*MAT((G),(2));
 COMMENT Notes:
 1. MAT((G),(2))/H would denote rightmultiplication by the
 inverse, which is not what we want.
 2. Solutions for a set of righthandside vectors are most
 efficiently computed simultaneously by collecting the right
 hand sides together as the columns of a single multiplecolumn
 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;
