Graham's number

Graham's number is connected to the following problem in Ramsey theory: In 1971, Graham and Rothschild proved the Graham–Rothschild theorem on the Ramsey theory of parameter words, a special case of which shows that this problem has a solution N*. They bounded the value of N* by , with N being a large but explicitly defined number :N=F^7(12) = F(F(F(F(F(F(F(12))))))), where F(n) = 2\uparrow^n 3 in Knuth's up-arrow notation; the number is between and in Conway chained arrow notation. This was reduced in 2014 via upper bounds on the Hales–Jewett number to :N' = 2 \uparrow\uparrow (2 \uparrow\uparrow (3 + 2 \uparrow\uparrow 8)), which contains three tetrations. In 2019 this was further improved to :N'' = (2 \uparrow\uparrow 5138) \cdot ((2 \uparrow\uparrow 5140) \uparrow\uparrow (2 \cdot 2 \uparrow\uparrow 5137)) \ll 2 \uparrow\uparrow (2 \uparrow\uparrow 5138). The lower bound of 6 was later improved to 11 by Geoffrey Exoo in 2003, and to 13 by Jerome Barkley in 2008. Thus, the best known bounds for N* are . Graham's number, G, is much larger than N: it is f^{64}(4), where f(n) = 3 \uparrow^n 3. This weaker upper bound for the problem, attributed to an unpublished work of Graham, was eventually published and named by Martin Gardner in Scientific American in November 1977. ==Publication==

The number gained a degree of popular attention when Martin Gardner described it in the "Mathematical Games" section of Scientific American in November 1977, writing that Graham had recently established, in an unpublished proof, "a bound so vast that it holds the record for the largest number ever used in a serious mathematical proof." The 1980 Guinness Book of World Records repeated Gardner's claim, adding to the popular interest in this number. According to physicist John Baez, Graham invented the quantity now known as Graham's number in conversation with Gardner. While Graham was trying to explain a result in Ramsey theory which he had derived with his collaborator Bruce Lee Rothschild, Graham found that the said quantity was easier to explain than the actual number appearing in the proof. Because the number which Graham described to Gardner is larger than the number in the paper itself, both are valid upper bounds for the solution to the problem studied by Graham and Rothschild. ==Definition==

Using Knuth's up-arrow notation, Graham's number G (as defined in Gardner's Scientific American article) is \left. \begin{matrix} G &=&3\underbrace{\uparrow \uparrow \cdots \cdots \cdots \cdots \cdots \uparrow}3 \\ & &3\underbrace{\uparrow \uparrow \cdots \cdots \cdots \cdots \uparrow}3 \\ & & \underbrace{\qquad \quad \vdots \qquad \quad} \\ & &3\underbrace{\uparrow \uparrow \cdots \cdots \uparrow}3 \\ & &3\uparrow \uparrow \uparrow \uparrow3 \end{matrix} \right \} \text{64 layers} where the number of arrows in each layer is specified by the value of the next layer below it; that is, G = g_{64}, where g_1=3\uparrow\uparrow\uparrow\uparrow 3, g_n = 3\uparrow^{g_{n-1}}3, and where a superscript on an up-arrow indicates how many arrows there are. In other words, G is calculated in 64 steps: the first step is to calculate g1 with four up-arrows between 3s; the second step is to calculate g2 with g1 up-arrows between 3s; the third step is to calculate g3 with g2 up-arrows between 3s; and so on, until finally calculating G = g64 with g63 up-arrows between 3s. Equivalently, G = f^{64}(4),\text{ where }f(n) = 3 \uparrow^n 3, and the superscript on f indicates an iteration of the function, e.g., f^4(n) = f(f(f(f(n)))). Expressed in terms of the family of hyperoperations \text{H}_0, \text{H}_1, \text{H}_2, \cdots, the function f is the particular sequence f(n) = \text{H}_{n+2}(3,3), which is a version of the rapidly growing Ackermann function A(n, n). (In fact, f(n) > A(n, n) for all n.) The function f can also be expressed in Conway chained arrow notation as f(n) = 3 \rightarrow 3 \rightarrow n, and this notation also provides the following bounds on G: 3\rightarrow 3\rightarrow 64\rightarrow 2 Magnitude To convey the difficulty of appreciating the enormous size of Graham's number, it may be helpful to express—in terms of exponentiation alone—just the first term (g1) of the rapidly growing 64-term sequence. First, in terms of tetration ( \uparrow\uparrow) alone: g_1 = 3 \uparrow \uparrow \uparrow \uparrow 3 = 3 \uparrow \uparrow \uparrow (3 \uparrow \uparrow \uparrow 3) = 3 \uparrow\uparrow (3 \uparrow\uparrow (3 \uparrow\uparrow \ \dots \ (3 \uparrow\uparrow 3) \dots )) where the number of 3s in the expression on the right is 3 \uparrow \uparrow \uparrow 3 = 3 \uparrow \uparrow (3 \uparrow \uparrow 3). Now each tetration (\uparrow\uparrow) operation reduces to a power tower (\uparrow) according to the definition 3 \uparrow\uparrow X = 3 \uparrow (3 \uparrow (3 \uparrow \dots (3 \uparrow 3) \dots)) = 3^{3^{\cdot^{\cdot^{\cdot^{3}}}}} where there are X 3s. Thus, g_1 = 3 \uparrow\uparrow (3 \uparrow\uparrow (3 \uparrow\uparrow \ \dots \ (3 \uparrow\uparrow 3) \dots )) \quad \text{where the number of 3s is} \quad 3 \uparrow \uparrow (3 \uparrow \uparrow 3) becomes, solely in terms of repeated "exponentiation towers", g_1 = \underbrace{ \left. \begin{matrix}3^{3^{\cdot^{\cdot^{\cdot^{\cdot^{3}}}}}}\end{matrix} \right \} \left. \begin{matrix}3^{3^{\cdot^{\cdot^{\cdot^{3}}}}}\end{matrix} \right \} \dots \left. \begin{matrix}3^{3^3}\end{matrix} \right \} 3}_{ \left. \begin{matrix}3^{3^{\cdot^{\cdot^{\cdot^{3}}}}}\end{matrix} \right \} \left. \begin{matrix}3^{3^3}\end{matrix} \right \} 3} and where the number of 3s in each tower, starting from the leftmost tower, is specified by the value of the next tower to the right. In other words, g1 is computed by first calculating the number of towers, n = 3\uparrow (3\uparrow (3\ \dots\ \uparrow 3)) (where the number of 3s is 3\uparrow (3\uparrow 3) = 7625597484987), and then computing the nth tower in the following sequence: 1st tower: 3 2nd tower: 3↑3↑3 (number of 3s is 3) = 7625597484987 3rd tower: 3↑3↑3↑3↑...↑3 (number of 3s is 7625597484987) = … ⋮ g1 = nth tower: 3↑3↑3↑3↑3↑3↑3↑...↑3 (number of 3s is given by the tower) where the number of 3s in each successive tower is given by the tower just before it. The result of calculating the third tower is the value of n, the number of towers for g1. The magnitude of this first term, g1, is so large that it is practically incomprehensible, even though the above display is relatively easy to comprehend. Even n, the mere number of towers in this formula for g1, is far greater than the number of Planck volumes (roughly 10185 of them) into which one can imagine subdividing the observable universe. And after this first term, still another 63 terms remain in the rapidly growing g sequence before Graham's number G = g64 is reached. To illustrate just how fast this sequence grows, while g1 is equal to 3 \uparrow \uparrow \uparrow \uparrow 3 with only four up arrows, the number of up arrows in g2 is this incomprehensibly large number g1. ==Mod n==

The residue of Graham's number mod n, starting with n = 1, is :0, 1, 0, 3, 2, 3, 6, 3, 0, 7, 9, 3, 1, 13, 12, 11, 7, 9, 18, 7, 6, 9, 18, 3, 12, 1, 0, 27, 10, 27, 23, 27, 9, 7, 27, 27, 36, 37, 27, 27, 27, 27, 2, 31, 27, 41, 6, 27, 6, 37, … ==References==