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Multiplication Table is an useful table to remember to help you learn multiplication by 42. You should also practice the examples given because the best way to learn is by doing, not memorizing. Online Practice What makes a number particularly interesting or uninteresting is a question that mathematician and psychologist Nicolas Gauvrit, computational natural scientist Hector Zenil and I have studied, starting with an analysis of the sequences in the OEIS. Aside from a theoretical connection to Kolmogorov complexity (which defines the complexity of a number by the length of its minimal description), we have shown that the numbers contained in Sloane’s encyclopedia point to a shared mathematical culture and, consequently, that OEIS is based as much on human preferences as pure mathematical objectivity. Problem of the Sum of Three Cubes To answer that question, mathematicians started by taking the nonprohibited values 1, 2, 3, 6, 7, 8, 9, 10, 11, 12, 15, 16 ... ( A060464 in OEIS) and examining them one by one. If solutions can be found for all those examined values, it will be reasonable to conjecture that for any integer n that is not of the form n = 9 m + 4 or n = 9 m + 5, there are solutions to the equation n = a 3 + b 3 + c 3. An inch (symbol: in) is a unit of length. It is defined as 1⁄12 of a foot, also is 1⁄36 of a yard. Though traditional standards for the exact length of an inch have varied, it is equal to exactly 25.4 mm. The inch is a popularly used customary unit of length in the United States, Canada, and the United Kingdom. Definition of centimeter

Like other computational number theorists who work in arithmetic geometry, he was aware of the “sum of three cubes” problem. And the two had worked together before, helping to build the L-functions and Modular Forms Database (LMFDB), an online atlas of mathematical objects related to what is known as the Langlands Program. “I was thrilled when Andy asked me to join him on this project,” says Sutherland. The cases of 165, 795 and 906 were also solved recently. For integers below 1,000, only 114, 390, 579, 627, 633, 732, 921 and 975 remain to be solved. The problem is stated as follows: What integers n can be written as the sum of three whole-number cubes ( n = a 3 + b 3 + c 3)? And for such integers, how do you find a, b and c ? As a practical matter, the difficulty in making this calculation is that for a given n, the space of the triplets to be considered involves negative integers. This triplet space is therefore infinite, unlike the computation for the sum of squares. For that particular problem, any solution has an absolute value lower than the square root of a given n. Moreover for the sum of squares, we know perfectly well what is possible and impossible. The number is the sum of the first three odd powers of two—that is, 2 1 + 2 3 + 2 5 = 42. It is an element in the sequence a( n), which is the sum of n odd powers of 2 for n> 0. The sequence corresponds to entry A020988 in The On-Line Encyclopedia of Integer Sequences (OEIS), created by mathematician Neil Sloane. In base 2, the nth element may be specified by repeating 10 n times (1010 ... 10). The formula for this sequence is a( n) = (2/3)(4 n– 1). As n increases, the density of numbers tends toward zero, which means that the numbers belonging to this list, including 42, are exceptionally rare. The number 42 also turns up in a whole string of curious coincidences whose significance is probably not worth the effort to figure out. For example:The difficulty appears so daunting that the question “Is n a sum of three cubes?” may be undecidable. In other words, no algorithm, however clever, may be able to process all possible cases. In 1936, for example, Alan Turing showed that no algorithm can solve the halting problem for every possible computer program. But here we are in a readily describable, purely mathematical domain. If we could prove such undecidability, that would be a novelty. Note that for some integer values of n, the equation n = a 3 + b 3 + c 3 has no solution. Such is the case for all integers n that are expressible as 9 m + 4 or 9 m + 5 for any integer m (e.g., 4, 5, 13, 14, 22, 23). Demonstrating this assertion is straightforward: we use the “modulo 9” (mod 9) calculation, which is equivalent to assuming that 9 = 0 and then manipulating only numbers between 0 and 8 or between −4 and 4. When we do so, we see that: In other words, the cube of an integer modulo 9 is –1 (= 8), 0 or 1. Adding any three numbers among these numbers gives: The conjecture that solutions exist for all integers n that are not of the form 9 m + 4 or 9 m + 5 would appear to be confirmed. In 1992 Roger Heath-Brown of the University of Oxford proposed a stronger conjecture stating that there are infinitely many ways to express all possible n’s as the sum of three cubes. The work is far from over.

In 2009, employing a method proposed by Noam Elkies of Harvard University in 2000, German mathematicians Andreas-Stephan Elsenhans and Jörg Jahnel explored all the triplets a, b, c of integers with an absolute value less than 10 14 to find solutions for n between 1 and 1,000. The paper reporting their findings concluded that the question of the existence of a solution for numbers below 1,000 remained open only for 33, 42, 74, 114, 165, 390, 579, 627, 633, 732, 795, 906, 921 and 975. For integers less than 100, just three enigmas remained: 33, 42 and 74. The number 42 is especially significant to fans of science fiction novelist Douglas Adams’ “The Hitchhiker’s Guide to the Galaxy, ” because that number is the answer given by a supercomputer to “the Ultimate Question of Life, the Universe, and Everything.”That calculation is not the only other solution. In 1936 German mathematician Kurt Mahler proposed an infinite number of them. For any integer p:

Apart from allusions to 42 deliberately introduced by computer scientists for fun and the inevitable encounters with it that crop up when you poke around a bit in history or the world, you might still wonder whether there is anything special about the number from a strictly mathematical point of view. Mathematically Unique? This is another reason I really liked running this computation on Charity Engine — we actually did use a planetary-scale computer to settle a longstanding open question whose answer is 42.”

Inches to centimeters formulae

A question that naturally follows is: Is there at least one solution for every nonprohibited value? Computers at Work You cannot get a sum of 4 or 5 (= –4). This restriction means that sums of three cubes are never numbers of the form 9 m + 4 or 9 m + 5. We thus say that n = 9 m + 4 and n = 9 m + 5 are prohibited values. Searching for Solutions The answer came in a 2020 preprint, the result of a huge computational effort coordinated by Booker and Andrew Sutherland of the Massachusetts Institute of Technology. Computers participating in the Charity Engine network of personal computers, calculating for the equivalent of more than one million hours, showed: Forty-two is a Catalan number. These numbers are extremely rare, much more so than prime numbers: only 14 of the former are lower than one billion. Catalan numbers were first mentioned, under another name, by Swiss mathematician Leonhard Euler, who wanted to know how many different ways an n-sided convex polygon could be cut into triangles by connecting vertices with line segments. The beginning of the sequence ( A000108 in OEIS) is 1, 1, 2, 5, 14, 42, 132.... The nth element of the sequence is given by the formula c( n) = (2 n)! / ( n!( n + 1)!). And like the two preceding sequences, the density of numbers is null at infinity.