The word inf represents the
mathematical concept of infinity. Calc actually has
three slightly different infinity-like values: inf,
uinf, and nan. These are just regular
variable names (see Variables); you should avoid using
these names for your own variables because Calc gives them
special treatment. Infinities, like all variable names, are
normally entered using algebraic entry.
Mathematically speaking, it is not rigorously correct to treat “infinity” as if it were a number, but mathematicians often do so informally. When they say that ‘1 / inf = 0’, what they really mean is that ‘1 / x’, as ‘x’ becomes larger and larger, becomes arbitrarily close to zero. So you can imagine that if ‘x’ got “all the way to infinity,” then ‘1 / x’ would go all the way to zero. Similarly, when they say that ‘exp(inf) = inf’, they mean that ‘exp(x)’ grows without bound as ‘x’ grows. The symbol ‘-inf’ likewise stands for an infinitely negative real value; for example, we say that ‘exp(-inf) = 0’. You can have an infinity pointing in any direction on the complex plane: ‘sqrt(-inf) = i inf’.
The same concept of limits can be used to define ‘1 / 0’. We really want the value that ‘1 / x’ approaches as ‘x’ approaches zero. But if all we have is ‘1 / 0’, we can't tell which direction ‘x’ was coming from. If ‘x’ was positive and decreasing toward zero, then we should say that ‘1 / 0 = inf’. But if ‘x’ was negative and increasing toward zero, the answer is ‘1 / 0 = -inf’. In fact, ‘x’ could be an imaginary number, giving the answer ‘i inf’ or ‘-i inf’. Calc uses the special symbol ‘uinf’ to mean undirected infinity, i.e., a value which is infinitely large but with an unknown sign (or direction on the complex plane).
Calc actually has three modes that say how infinities are
handled. Normally, infinities never arise from calculations that
didn't already have them. Thus, ‘1
/ 0’ is treated simply as an error and left
unevaluated. The m i (calc-infinite-mode)
command (see Infinite
Mode) enables a mode in which ‘1 / 0’ evaluates to uinf
instead. There is also an alternative type of infinite mode which
says to treat zeros as if they were positive, so that
‘1 / 0 = inf’.
While this is less mathematically correct, it may be the answer
you want in some cases.
Since all infinities are “as large” as all others,
Calc simplifies, e.g., ‘5
inf’ to ‘inf’. Another example is
‘5 - inf =
-inf’, where the ‘-inf’ is so large that adding a finite
number like five to it does not affect it. Note that
‘a - inf’ also
results in ‘-inf’; Calc assumes that variables
like a always stand for finite quantities. Just to
show that infinities really are all the same size, note that
‘sqrt(inf) = inf^2 = exp(inf) =
inf’ in Calc's notation.
It's not so easy to define certain formulas like
‘0 * inf’ and
‘inf / inf’.
Depending on where these zeros and infinities came from, the
answer could be literally anything. The latter formula could be
the limit of ‘x /
x’ (giving a result of one), or
‘2 x / x’
(giving two), or ‘x^2 /
x’ (giving inf), or
‘x / x^2’
(giving zero). Calc uses the symbol nan to represent
such an indeterminate value. (The name
“nan” comes from analogy with the “NAN”
concept of IEEE standard arithmetic; it stands for “Not A
Number.” This is somewhat of a misnomer, since
nan does stand for some number or infinity,
it's just that which number it stands for cannot be
determined.) In Calc's notation, ‘0 * inf = nan’ and
‘inf / inf =
nan’. A few other common indeterminate
expressions are ‘inf -
inf’ and ‘inf
^ 0’. Also, ‘0
/ 0 = nan’ if you have turned on Infinite
mode (as described above).
Infinities are especially useful as parts of intervals. See Interval Forms.