Professor of Mathematics (Emeritus)

Loyola Marymount University

Los Angeles, CA

This graphing calculator carries over 50 significant digits of precision, and has a large

number of available functions.

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This page gives a brief overview of the functions in Calc-50, version 1.1.

For some more advanced examples of Calc-50 programming and graphics, see the

Calc-50 examples page.

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Calc-50 uses "RPN" input format, like classic scientific calculators such as the HP-15C.

This means the arguments are entered first, followed by the function.
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For example, 2 + 3 is computed by pressing these keys: 2, enter, 3, +
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These input values are kept in a 4-number stack. They are named x, y, z, t, where x is the

value displayed at the top of the screen. After 2, enter, 3 in the example above, y = 2 and

x = 3. Then the + operation adds x + y leaving 5 in x.

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There are 101 storage registers, numbered 0 to 100, where values can be saved.

6, sto stores x in register 6, and 42, rcl recalls the value in register 42 and puts it into x.

Some fancier applications can use "index registers", as in 1, rcl, sto. This stores the

current x into whichever register is given by the value in register 1.
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The fix key sets the number of digits after the decimal in the display. If x is 123.4567,

then 2, fix will display it as 123.46. The value of x is not changed, only the display

is different.
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The sci key sets the number of significant digits to display in scientific notation.

If x is 123.4567, then 5, sci will display it as 1.2346e+2.

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The display can show up to 50 digits. In fix mode, if x is too large or too
small for the

display, it will be shown in scientific mode.
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Pressing the help key followed by another key brings up a screen like this one.
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Each key on the calculator has an associated help screen with example usage, domain, and

sometimes a graph. Some general notes about Calc-50 can be found on the help screens for

digits 0, 1, 2, ....
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The exit key then returns to the normal calculator screen.
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The func key brings up one of 9 different sets of functions. The 20 keys in the lower left

change with each screen.
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Pressing func by itself scrolls to the next screen, or pressing a digit from 1 to 9 followed

by func goes directly to that screen.
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Screen 1 has the basic math functions: trig, exponentials,
logs and powers.
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Screen 2 has rounding to integers, max/min, random numbers, some statistical functions,

mod, power mod, gcd, and rational approximation:

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Screen 3 has factorial, gamma function, log gamma, combinations, permutations, the Euler

gamma constant, Bernoulli numbers, beta, psi, polygamma, pochhammer, error function,

incomplete gamma, incomplete beta. Also conversion between rectangular and polar form,

and absolute value.
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Screen 4 has the exponential integral, sin, cos, sinh, cosh, and log integrals, Fresnel C and

S functions, Bessel J and Y functions.

Also time and calendar conversions, day of the week, percents,
hypotenuse, and last x.
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Screen 5 has some financial functions that solve for various variables from a home

or auto loan.
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After entering any 4 of the 5 values n (number of payments),
i (interest rate),

pv (present value), pmt (payment), fv (future value) using the store
keys, pressing the

remaining key computes that value.
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Screen 6 has the keys used to define and edit user functions, and also 7 keys that use those

functions: derivative, nth derivative, maximum f(x), minimum f(x), integral of f(x) from

a to b, arc length of f(x) from a to b, solve f(x) = 0.
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The integration and arc length keys try to get at least 40 digits correct, and the others try

for at least 50 digits.
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Depending on f(x), these functions can be slow. When a slow function is running, a

small spinning wheel is displayed above the help key and "running ..." appears in the

calculator output area to show that the function is still working.
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The time key can be used to limit the running time for these functions.
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Factoring integers with the fctr key is usually fast for integers up to about 30 or 40 digits.

Inputs up to 50 digits are allowed, and most of these can be factored, but some are slow.

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The prog key allows for up to 10 user functions to be defined. Press a digit 0-9 followed

by prog to enter program mode and define a new function or edit an existing one.

For example, define function 6 to be cos( ci(x) ).
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6, prog, clr f, 4, func, ci, 1, func, cos, 6, func, end
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In program mode, before the end key is pressed, here is how the function
definition looks:
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The underscore _ after cos is the cursor. The 4 arrow keys move the cursor so a function

can be edited. The copy and paste keys let a function be copied and then pasted into another

function. The clrf key clears the function being edited, and bksp is a backspace that deletes

the operation immediately to the left of the cursor.
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Pressing the end key puts the calculator back in normal mode. To evaluate function 6 at

x = 1.2 and compute cos( ci(1.2) ):
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1.2, enter, 6, fn
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With the calculator in radian mode, this function has a root near x = 0.117. To use the solve

key to find it, starting with 0.1 as an initial guess,
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0.1, enter, 6, solv
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The result is 0.117116.... Checking this by computing the function at this value (6, fn)

gives about -2.5e-57, indicating the root should be accurate to over 50 digits.
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For another example, integrate this function from 1 to 3. To use the integrate key,
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1, enter, 3, enter, 6, integrate
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This numerical integration takes less than a second. The result is 1.86081....

Pressing x<->y to exchange the x and y registers shows y was about 2.03e-42, meaning

the integration algorithm stopped with the last two iterations agreeing to about 42 digits.
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Screen 7 has more keys that involve user functions: graphics, limits, sums, products,

differential equations, and function selection.
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The sum key can be used to do more than just simple sums. It can provide general loop

control for making lists and iterating recurrence relations.

For examples of this kind of programming, including some graphics applications, see the

Calc-50 examples page

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Four kinds of graphs are available: y = f(x) plots, parametric ( x(t), y(t) ) graphs,

polar r = f(theta) graphs, and list plots. Up to five graphs can be displayed at once

by using the add plot, ..., keys to add another graph to previously defined graphs.

Other keys can override the default settings for max and min y-values for the display

window, spacing for x and y axis tick marks, graph aspect ratio, horizontal and vertical

grid lines.
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Use the plot key to graph f(x) = sin(3x) + cos(2x) from 0 to pi
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7, prog, 1, sto, 3, *, sin, 1, rcl, 2, *, cos, +, end
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0, pi, 7, plot
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Touching the screen in the graph area puts a small symbol on the screen at that location

and displays the (x,y) coordinates of that position at the botton of the screen. Two buttons

appear next to the exit button, "zoom in" and "zoom out".
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Zooming in re-draws the graph centered at the x-value shown at the bottom, with an

x-interval half as big as before. Zooming out does the same, with an x-interval twice

as big as before.
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Screen 8 has unit conversion functions. Pressing a conversion key changes the value in the

x register from the units at the top of the key to the units at the bottom. Pressing the

inv key followed by a conversion key converts the other direction. The units of the result

are displayed along with its numerical value.
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Screen 9 puts the calculator into complex arithmetic mode. The complex elementary

functions and basic arithmetic operations are provided.
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To enter 1 + 2i, press

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1, +i, 2, enter
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Fewer digits can be displayed in complex mode for the real and imaginary parts of the

number, but each part still has over 50-digit accuracy internally.
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The complex stack x,y,z,t is separate from the real stack used for the other screens.

Doing some calculations in screen 9 will leave the real stack values unchanged when

going back to one the the other screens.
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The complex sto and rcl keys use 2 storage registers to hold one complex number.

5, sto while in complex mode will store the real part of x in register 5 and the imaginary

part in register 6. This can be used to communicate values between real and complex

modes, since registers 5 and 6 can be accessed separately from any real mode screen.
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Why would anyone need 50-digit accuracy?
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Most of the time, we don't. Most calculators have 14 to 16 digit accuracy, and exponent

ranges up to about 1.0e+1000. But some calculations need more accuracy or greater range.

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Some of the most common applications for high-precision calculations are deriving formulas

to be used in ordinary computer applications. If we want the coefficients in a formula to

be correctly rounded in the machine's double precision, then they usually must be computed

using higher precision.
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An example of this is the derivation of the
Guass quadrature rule on the Calc-50 examples

page. If we used only double precision to do the derivation, the coefficients needed in the

formula would not be accurate to full precision.
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Sometimes it can be hard to see that a formula is unstable and needs more accuracy, so it

is nice to have what seems like excessive precision available.
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Here is a simple example. Examine the behavior of this function as x approaches zero.
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f(x) = ( tan(sin(x)) - sin(tan(x)) ) / x^7
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Doing f(0.03) on a standard calculator (TI-89) gives 0.033379, which is correct to only

3 significant digits.The older HP-15C calculator does even worse. It gives 0.0, no correct

digits at all.
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The same calculation using Calc-50 gives .033367877648658374713095209129 when

displaying 30 digits. The formula is still losing accuracy, but by carrying 50 digits we can

afford to lose 11 digits in this unstable formula and still have plenty of accuracy left.
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Getting closer to zero makes things even worse for the TI-89. f(0.001) gives -100000.0

while Calc-50 shows .033333371693146997367349091810, still accurate to 30 places.
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In 2016, the built-in calculator app for the iPhone does not have many mathematical

functions, but it does use its compiler's quadruple precision to provide about 34 digits.

It can get f(0.001) correct to 13 digits, but f(0.00001) has only 1 significant digit.
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Calc-50 is starting to notice the instability for f(0.00001), getting "only" 27 digits correct.

For extreme cases like this, Calc-50 has one other trick for overcoming instability. The nth

derivative key on screen 6 works with derivative formulas that are also unstable, so it raises

precision above the user's 50-digit level to return accurate derivatives.
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Defining f(x) as a user function on screen 6, the nth derivative key allows n = 0, meaning

it will evaluate our unstable function at higher precision. That gives f(0.00001) correct

to 50 digits.
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Here is a second example. This time it is the exponent range that causes problems. Find

the probability of getting exactly one million heads if we toss a fair coin two million times.

This formula is
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comb( 2e6 , 1e6 ) * (0.5)^2e6
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where comb is the combination function, also called a binomial coefficient, and the second

term is 1/2 to the power two million.
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The answer is about 0.000564, and this formula is not unstable. The problems for standard

calculators are that not all of them have a combination function, and the two numbers
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comb( 2e6 , 1e6 ) = 5.5304e+602056 and (0.5)^2e6 = 1.0202e-602060
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are far outside the exponent range of a normal calculator.

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Summing up, some of the advantages Calc-50 over other calculators are:

- Many more functions available

- High precision means the user doesn't have to be as worried about moderately unstable

formulas losing too much precision

- Greater exponent range means fewer calculations fail because intermediate results

became too large or small

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