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#### 11.3.1 Default Simplifications

This section describes the “default simplifications,” those which are normally applied to all results. For example, if you enter the variable ‘`x`’ on the stack twice and push `+`, Calc's default simplifications automatically change ‘`x + x`’ to ‘`2 x`’.

The `m O` command turns off the default simplifications, so that ‘`x + x`’ will remain in this form unless you give an explicit “simplify” command like `=` or `a v`. See Algebraic Manipulation. The `m D` command turns the default simplifications back on.

The most basic default simplification is the evaluation of functions. For example, ‘`2 + 3`’ is evaluated to ‘`5`’, and ‘``sqrt`(9)`’ is evaluated to ‘`3`’. Evaluation does not occur if the arguments to a function are somehow of the wrong type ‘``tan`([2,3,4])`’), range (‘``tan`(90)`’), or number (‘``tan`(3,5)`’), or if the function name is not recognized (‘``f`(5)`’), or if Symbolic mode (see Symbolic Mode) prevents evaluation (‘``sqrt`(2)`’).

Calc simplifies (evaluates) the arguments to a function before it simplifies the function itself. Thus ‘``sqrt`(5+4)`’ is simplified to ‘``sqrt`(9)`’ before the `sqrt` function itself is applied. There are very few exceptions to this rule: `quote`, `lambda`, and `condition` (the `::` operator) do not evaluate their arguments, `if` (the `? :` operator) does not evaluate all of its arguments, and `evalto` does not evaluate its lefthand argument.

Most commands apply the default simplifications to all arguments they take from the stack, perform a particular operation, then simplify the result before pushing it back on the stack. In the common special case of regular arithmetic commands like `+` and `Q` [`sqrt`], the arguments are simply popped from the stack and collected into a suitable function call, which is then simplified (the arguments being simplified first as part of the process, as described above).

The default simplifications are too numerous to describe completely here, but this section will describe the ones that apply to the major arithmetic operators. This list will be rather technical in nature, and will probably be interesting to you only if you are a serious user of Calc's algebra facilities.

As well as the simplifications described here, if you have stored any rewrite rules in the variable `EvalRules` then these rules will also be applied before any built-in default simplifications. See Automatic Rewrites, for details.

And now, on with the default simplifications:

Arithmetic operators like `+` and `*` always take two arguments in Calc's internal form. Sums and products of three or more terms are arranged by the associative law of algebra into a left-associative form for sums, ‘`((a + b) + c) + d`’, and (by default) a right-associative form for products, ‘`a * (b * (c * d))`’. Formulas like ‘`(a + b) + (c + d)`’ are rearranged to left-associative form, though this rarely matters since Calc's algebra commands are designed to hide the inner structure of sums and products as much as possible. Sums and products in their proper associative form will be written without parentheses in the examples below.

Sums and products are not rearranged according to the commutative law (‘`a + b`’ to ‘`b + a`’) except in a few special cases described below. Some algebra programs always rearrange terms into a canonical order, which enables them to see that ‘`a b + b a`’ can be simplified to ‘`2 a b`’. Calc assumes you have put the terms into the order you want and generally leaves that order alone, with the consequence that formulas like the above will only be simplified if you explicitly give the `a s` command. See Algebraic Simplifications.

Differences ‘`a - b`’ are treated like sums ‘`a + (-b)`’ for purposes of simplification; one of the default simplifications is to rewrite ‘`a + (-b)`’ or ‘`(-b) + a`’, where ‘`-b`’ represents a “negative-looking” term, into ‘`a - b`’ form. “Negative-looking” means negative numbers, negated formulas like ‘`-x`’, and products or quotients in which either term is negative-looking.

Other simplifications involving negation are ‘`-(-x)`’ to ‘`x`’; ‘`-(a b)`’ or ‘`-(a/b)`’ where either ‘`a`’ or ‘`b`’ is negative-looking, simplified by negating that term, or else where ‘`a`’ or ‘`b`’ is any number, by negating that number; ‘`-(a + b)`’ to ‘`-a - b`’, and ‘`-(b - a)`’ to ‘`a - b`’. (This, and rewriting ‘`(-b) + a`’ to ‘`a - b`’, are the only cases where the order of terms in a sum is changed by the default simplifications.)

The distributive law is used to simplify sums in some cases: ‘`a x + b x`’ to ‘`(a + b) x`’, where ‘`a`’ represents a number or an implicit 1 or -1 (as in ‘`x`’ or ‘`-x`’) and similarly for ‘`b`’. Use the `a c`, `a f`, or `j M` commands to merge sums with non-numeric coefficients using the distributive law.

The distributive law is only used for sums of two terms, or for adjacent terms in a larger sum. Thus ‘`a + b + b + c`’ is simplified to ‘`a + 2 b + c`’, but ‘`a + b + c + b`’ is not simplified. The reason is that comparing all terms of a sum with one another would require time proportional to the square of the number of terms; Calc relegates potentially slow operations like this to commands that have to be invoked explicitly, like `a s`.

Finally, ‘`a + 0`’ and ‘`0 + a`’ are simplified to ‘`a`’. A consequence of the above rules is that ‘`0 - a`’ is simplified to ‘`-a`’.

The products ‘`1 a`’ and ‘`a 1`’ are simplified to ‘`a`’; ‘`(-1) a`’ and ‘`a (-1)`’ are simplified to ‘`-a`’; ‘`0 a`’ and ‘`a 0`’ are simplified to ‘`0`’, except that in Matrix mode where ‘`a`’ is not provably scalar the result is the generic zero matrix ‘`idn(0)`’, and that if ‘`a`’ is infinite the result is ‘`nan`’.

Also, ‘`(-a) b`’ and ‘`a (-b)`’ are simplified to ‘`-(a b)`’, where this occurs for negated formulas but not for regular negative numbers.

Products are commuted only to move numbers to the front: ‘`a b 2`’ is commuted to ‘`2 a b`’.

The product ‘`a (b + c)`’ is distributed over the sum only if ‘`a`’ and at least one of ‘`b`’ and ‘`c`’ are numbers: ‘`2 (x + 3)`’ goes to ‘`2 x + 6`’. The formula ‘`(-a) (b - c)`’, where ‘`-a`’ is a negative number, is rewritten to ‘`a (c - b)`’.

The distributive law of products and powers is used for adjacent terms of the product: ‘`x^a x^b`’ goes to ‘`x^(a+b)`’ where ‘`a`’ is a number, or an implicit 1 (as in ‘`x`’), or the implicit one-half of ‘``sqrt`(x)`’, and similarly for ‘`b`’. The result is written using ‘`sqrt`’ or ‘`1/sqrt`’ if the sum of the powers is ‘`1/2`’ or ‘`-1/2`’, respectively. If the sum of the powers is zero, the product is simplified to ‘`1`’ or to ‘`idn(1)`’ if Matrix mode is enabled.

The product of a negative power times anything but another negative power is changed to use division: ‘`x^(-2) y`’ goes to ‘`y / x^2`’ unless Matrix mode is in effect and neither ‘`x`’ nor ‘`y`’ are scalar (in which case it is considered unsafe to rearrange the order of the terms).

Finally, ‘`a (b/c)`’ is rewritten to ‘`(a b)/c`’, and also ‘`(a/b) c`’ is changed to ‘`(a c)/b`’ unless in Matrix mode.

Simplifications for quotients are analogous to those for products. The quotient ‘`0 / x`’ is simplified to ‘`0`’, with the same exceptions that were noted for ‘`0 x`’. Likewise, ‘`x / 1`’ and ‘`x / (-1)`’ are simplified to ‘`x`’ and ‘`-x`’, respectively.

The quotient ‘`x / 0`’ is left unsimplified or changed to an infinite quantity, as directed by the current infinite mode. See Infinite Mode.

The expression ‘`a / b^(-c)`’ is changed to ‘`a b^c`’, where ‘`-c`’ is any negative-looking power. Also, ‘`1 / b^c`’ is changed to ‘`b^(-c)`’ for any power ‘`c`’.

Also, ‘`(-a) / b`’ and ‘`a / (-b)`’ go to ‘`-(a/b)`’; ‘`(a/b) / c`’ goes to ‘`a / (b c)`’; and ‘`a / (b/c)`’ goes to ‘`(a c) / b`’ unless Matrix mode prevents this rearrangement. Similarly, ‘`a / (b:c)`’ is simplified to ‘`(c:b) a`’ for any fraction ‘`b:c`’.

The distributive law is applied to ‘`(a + b) / c`’ only if ‘`c`’ and at least one of ‘`a`’ and ‘`b`’ are numbers. Quotients of powers and square roots are distributed just as described for multiplication.

Quotients of products cancel only in the leading terms of the numerator and denominator. In other words, ‘`a x b / a y b`’ is cancelled to ‘`x b / y b`’ but not to ‘`x / y`’. Once again this is because full cancellation can be slow; use `a s` to cancel all terms of the quotient.

Quotients of negative-looking values are simplified according to ‘`(-a) / (-b)`’ to ‘`a / b`’, ‘`(-a) / (b - c)`’ to ‘`a / (c - b)`’, and ‘`(a - b) / (-c)`’ to ‘`(b - a) / c`’.

The formula ‘`x^0`’ is simplified to ‘`1`’, or to ‘`idn(1)`’ in Matrix mode. The formula ‘`0^x`’ is simplified to ‘`0`’ unless ‘`x`’ is a negative number, complex number or zero. If ‘`x`’ is negative, complex or ‘`0.0`’, ‘`0^x`’ is an infinity or an unsimplified formula according to the current infinite mode. The expression ‘`0^0`’ is simplified to ‘`1`’.

Powers of products or quotients ‘`(a b)^c`’, ‘`(a/b)^c`’ are distributed to ‘`a^c b^c`’, ‘`a^c / b^c`’ only if ‘`c`’ is an integer, or if either ‘`a`’ or ‘`b`’ are nonnegative real numbers. Powers of powers ‘`(a^b)^c`’ are simplified to ‘`a^(b c)`’ only when ‘`c`’ is an integer and ‘`b c`’ also evaluates to an integer. Without these restrictions these simplifications would not be safe because of problems with principal values. (In other words, ‘`((-3)^1:2)^2`’ is safe to simplify, but ‘`((-3)^2)^1:2`’ is not.) See Declarations, for ways to inform Calc that your variables satisfy these requirements.

As a special case of this rule, ‘``sqrt`(x)^n`’ is simplified to ‘`x^(n/2)`’ only for even integers ‘`n`’.

If ‘`a`’ is known to be real, ‘`b`’ is an even integer, and ‘`c`’ is a half- or quarter-integer, then ‘`(a^b)^c`’ is simplified to ‘``abs`(a^(b c))`’.

Also, ‘`(-a)^b`’ is simplified to ‘`a^b`’ if ‘`b`’ is an even integer, or to ‘`-(a^b)`’ if ‘`b`’ is an odd integer, for any negative-looking expression ‘`-a`’.

Square roots ‘``sqrt`(x)`’ generally act like one-half powers ‘`x^1:2`’ for the purposes of the above-listed simplifications.

Also, note that ‘`1 / x^1:2`’ is changed to ‘`x^(-1:2)`’, but ‘`1 / `sqrt`(x)`’ is left alone.

Generic identity matrices (see Matrix Mode) are simplified by the following rules: ‘``idn`(a) + b`’ to ‘`a + b`’ if ‘`b`’ is provably scalar, or expanded out if ‘`b`’ is a matrix; ‘``idn`(a) + `idn`(b)`’ to ‘``idn`(a + b)`’; ‘`-`idn`(a)`’ to ‘``idn`(-a)`’; ‘`a `idn`(b)`’ to ‘``idn`(a b)`’ if ‘`a`’ is provably scalar, or to ‘`a b`’ if ‘`a`’ is provably non-scalar; ‘``idn`(a) `idn`(b)`’ to ‘``idn`(a b)`’; analogous simplifications for quotients involving `idn`; and ‘``idn`(a)^n`’ to ‘``idn`(a^n)`’ where ‘`n`’ is an integer.

The `floor` function and other integer truncation functions vanish if the argument is provably integer-valued, so that ‘``floor`(`round`(x))`’ simplifies to ‘``round`(x)`’. Also, combinations of `float`, `floor` and its friends, and `ffloor` and its friends, are simplified in appropriate ways. See Integer Truncation.

The expression ‘``abs`(-x)`’ changes to ‘``abs`(x)`’. The expression ‘``abs`(`abs`(x))`’ changes to ‘``abs`(x)`’; in fact, ‘``abs`(x)`’ changes to ‘`x`’ or ‘`-x`’ if ‘`x`’ is provably nonnegative or nonpositive (see Declarations).

While most functions do not recognize the variable `i` as an imaginary number, the `arg` function does handle the two cases ‘``arg`(`i`)`’ and ‘``arg`(-`i`)`’ just for convenience.

The expression ‘``conj`(`conj`(x))`’ simplifies to ‘`x`’. Various other expressions involving `conj`, `re`, and `im` are simplified, especially if some of the arguments are provably real or involve the constant `i`. For example, ‘``conj`(a + b i)`’ is changed to ‘``conj`(a) - `conj`(b) i`’, or to ‘`a - b i`’ if ‘`a`’ and ‘`b`’ are known to be real.

Functions like `sin` and `arctan` generally don't have any default simplifications beyond simply evaluating the functions for suitable numeric arguments and infinity. The `a s` command described in the next section does provide some simplifications for these functions, though.

One important simplification that does occur is that ‘``ln`(`e`)`’ is simplified to 1, and ‘``ln`(`e`^x)`’ is simplified to ‘`x`’ for any ‘`x`’. This occurs even if you have stored a different value in the Calc variable ‘`e`’; but this would be a bad idea in any case if you were also using natural logarithms!

Among the logical functions, `!(`a `<=` b`)` changes to a `>` b and so on. Equations and inequalities where both sides are either negative-looking or zero are simplified by negating both sides and reversing the inequality. While it might seem reasonable to simplify ‘`!!x`’ to ‘`x`’, this would not be valid in general because ‘`!!2`’ is 1, not 2.

Most other Calc functions have few if any default simplifications defined, aside of course from evaluation when the arguments are suitable numbers.