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2017 is not just another prime number

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41–50 of 63 posts

Re: 2017 is not just another prime number

#41

Earlier quoted context omitted.

The greedy algorithm works here: 2017 = 12^3 + 6^3 + 4^3 + 2^3 + 1^3

Technical note: 5 cubes is not enough for every number, so this property of 2017 is not trivial. From > Every positive integer can be written as the sum of nine (or fewer) positive cubes. This upper limit of nine cubes cannot be reduced because, for example, 23 cannot be written as the sum of fewer than nine positive cubes: > 23 = 2^3 + 2^3 + 1^3 + 1^3 + 1^3 + 1^3 + 1^3 + 1^3 + 1^3. I couldn't find if it's common tha…

http://oeis.org/A003328: Numbers that are the sum of 5 positive cubes

5, 12, 19, 26, 31, 33, 38, 40, 45, 52, 57, 59, 64, 68, 71, 75, 78, 82, 83, 89, 90, 94, 96, 97, 101, 108, 109, 115, 116, 120, 127, 129, 131, 134, 135, 136, 138, 143, 145, 146, 150, 152, 153, 155, 157, 162, 164, 169, 171, 172, 176, 181, 183, 188, 190, 192, 194

It seems this is fairly common (1757 is the 1000th such number), but of course that says nothing.

Reading http://mathworld.wolfram.com/CubicNumber.html, it is true that every sufficiently large integer is a sum of no more than 7 positive cubes.

It also states ”the only integers requiring nine positive cubes are 23 and 239. Wieferich proved that only 15 integers require eight cubes: 15, 22, 50, 114, 167, 175, 186, 212, 231, 238, 303, 364, 420, 428, and 454 (OEIS A018889).”

Even stronger (same page): ”Deshouillers et al. (2000) conjectured that 7373170279850 is the largest integer that cannot be expressed as the sum of four nonnegative cubes” (nice title for a paper: ”7 373 170 279 850.”. See http://www.ams.org/journals/mcom/2000-69-229/S0025-5718-99-0...)

If that is true, it is indeed common that 5 cubes is enough (since 4 almost always would be sufficient)

Re: 2017 is not just another prime number

#43
post #6

Earlier quoted context omitted.

Every number is special. Proof is by contradiction: Assume that not every number is mathematically interesting and let X be the first such number. However, the fact that X is the lowest such number is itself pretty special, right?

Well the first number that doesn't appear in OEIS is 18159. That make it interesting to me , but apparently this number isn't interesting to mathematicians. Of course since OEIS is finite, there will always be numbers which don't make it. What's really interesting is that some numbers are disproportionately underrepresented. They appear much less often than other numbers of the same size. And if you plot each number…

”Of course since OEIS is finite, there will always be numbers which don't make it”.

18159 appears in OEIS, but its search interface isn’t perfect.

For example it appears in http://oeis.org/A000027: ”The positive integers. Also called the natural numbers, the whole numbers or the counting numbers, but these terms are ambiguous.”

Re: 2017 is not just another prime number

#45
post #23

Meh. A prime number year last happened in 2011. Just kidding... happy new prime number year! BTW I'm really looking forward to the next perfect square year: 2025 (45^2). It last happened in 1936, and won't happen again until 2116.

I'm hoping to to see the only power of two in a millennium: 2048

Re: 2017 is not just another prime number

#46
post #6

Earlier quoted context omitted.

Every number is special. Proof is by contradiction: Assume that not every number is mathematically interesting and let X be the first such number. However, the fact that X is the lowest such number is itself pretty special, right?

This "proof" seems to be popular, and it always bothers me that it's invalid. It uses self-reference in an invalid way. Allow me to formalize; we take as a rigorous definition of an "interesting number" that a number has a unique property. Specifically, a number n is interesting if there is some predicate P(x) which is true only for n. In formal first order logic, n is interesting if there exist a predicate P and a n…

> Specifically, a number n is interesting if there is some predicate P(x) which is true only for n.

Well, if you read the "every number is interesting" "proof", this clearly doesn't capture the proof's criteria of interestingness.

I see it as analogous to Berry's paradox - the proof isn't "wrong" per se, but the relevant notion of interestingness is not well defined

Re: 2017 is not just another prime number

#47
post #6

Earlier quoted context omitted.

What I like to do to measure the "mathematical interestingness" of a number, is check how many times it appears in OEIS. A database of sequences of numbers found in mathematical research. 2017 appeared in 453 sequences. For comparison; 2016 appears 833 sequences, and 2018 appears in 113. http://oeis.org/search?q=seq%3A2016&sort=&language=english&g...

Every number is special. Proof is by contradiction: Assume that not every number is mathematically interesting and let X be the first such number. However, the fact that X is the lowest such number is itself pretty special, right?

What about the second such number?

(Waiting for the new OEIS sequence of uninteresting numbers.)

Re: 2017 is not just another prime number

#48
post #6

Earlier quoted context omitted.

Every number is special. Proof is by contradiction: Assume that not every number is mathematically interesting and let X be the first such number. However, the fact that X is the lowest such number is itself pretty special, right?

This "proof" seems to be popular, and it always bothers me that it's invalid. It uses self-reference in an invalid way. Allow me to formalize; we take as a rigorous definition of an "interesting number" that a number has a unique property. Specifically, a number n is interesting if there is some predicate P(x) which is true only for n. In formal first order logic, n is interesting if there exist a predicate P and a n…

”Specifically, a number n is interesting if there is some predicate P(x) which is true only for n”

That’s not that good a definition. Since the predicate “P(x) = x = 4578634986” is only true for x = 4578634986, would that imply that 4578634986 is interesting?

Re: 2017 is not just another prime number

#49
post #8
post #6

Earlier quoted context omitted.

Every number is special. Proof is by contradiction: Assume that not every number is mathematically interesting and let X be the first such number. However, the fact that X is the lowest such number is itself pretty special, right?

Can this proof be adapted for the reals, or is it only the case that every integer is special?

No. ”Assume that not every number is mathematically interesting and let X be the first such number” doesn’t work as is because a set of reals need not contain a smallest number. For example, the set

   {x | ∃ n ∈ ℕ : 10^n = 1}

   =

   {1, 0.1, 0.01, 0.001, 0.0001, 0.00001, …}
doesn’t have a smallest number.

It is impossible to adapt this proof because the set of reals is uncountable. It is doable for the rationals, though, as they _are_ countable.

Re: 2017 is not just another prime number

#50
post #43

Earlier quoted context omitted.

Well the first number that doesn't appear in OEIS is 18159. That make it interesting to me , but apparently this number isn't interesting to mathematicians. Of course since OEIS is finite, there will always be numbers which don't make it. What's really interesting is that some numbers are disproportionately underrepresented. They appear much less often than other numbers of the same size. And if you plot each number…

”Of course since OEIS is finite, there will always be numbers which don't make it” . 18159 appears in OEIS, but its search interface isn’t perfect. For example it appears in http://oeis.org/A000027 : ”The positive integers. Also called the natural numbers, the whole numbers or the counting numbers, but these terms are ambiguous.”

Yes 18159 does fit the definition of a natural number. But OEIS only contains the first few terms of every sequence. So I'm technically correct that it isn't in OEIS. And I would argue that being the 10k+ term in a sequence doesn't qualify as interesting.
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