Rules
There are many variations of four fours; their primary difference is which mathematical symbols are allowed. Essentially all variations at least allow addition ("+"), subtraction ("−"), multiplication ("×"), division ("÷"), and parentheses, as well as concatenation (e.g., "44" is allowed). Most also allow the factorial ("!"), exponentiation (e.g. "444"), the decimal point (".") and the square root ("√") operation, although sometimes square root is specifically excluded on the grounds that there is an implied "2" for the second root. Other operations allowed by some variations include subfactorial, ("!" before the number: !4 equals 9), overline (an infinitely repeated digit), an arbitrary root power, the gamma function (Γ, where Γ(x) = (x − 1)!), and percent ("%"). Thus 4/4% = 100 and Γ(4)=6. A common use of the overline in this problem is for this value:
Typically the "log" operators are not allowed, since there is a way to trivially create any number using them. Paul Bourke credits Ben Rudiak-Gould with this description of how natural logarithms (ln) can be used to represent any positive integer n as:
Additional variants (usually no longer called "four fours") replace the set of digits ("4, 4, 4, 4") with some other set of digits, say of the birthyear of someone. For example, a variant using "1975" would require each expression to use one 1, one 9, one 7, and one 5.
Read more about this topic: Four Fours
Famous quotes containing the word rules:
“Now that the steam engine rules the world, a title is an absurdity, still I am all dressed up in this title. It will crush me if I do not support it. The title attracts attention to myself.”
—Stendhal [Marie Henri Beyle] (1783–1842)
“There is no country in which so absolute a homage is paid to wealth. In America there is a touch of shame when a man exhibits the evidences of large property, as if after all it needed apology. But the Englishman has pure pride in his wealth, and esteems it a final certificate. A coarse logic rules throughout all English souls: if you have merit, can you not show it by your good clothes and coach and horses?”
—Ralph Waldo Emerson (1803–1882)
“When I hear the hypercritical quarreling about grammar and style, the position of the particles, etc., etc., stretching or contracting every speaker to certain rules of theirs ... I see that they forget that the first requisite and rule is that expression shall be vital and natural, as much as the voice of a brute or an interjection: first of all, mother tongue; and last of all, artificial or father tongue. Essentially your truest poetic sentence is as free and lawless as a lamb’s bleat.”
—Henry David Thoreau (1817–1862)