Wikipedia:Reference desk/Archives/Mathematics/2014 June 1

= June 1 =

k=ab+ac+bc, max k not possible?
Is there a maximum k where k is not expressible as ab+bc+ac where a, b and c are positive integers?Naraht (talk) 03:48, 1 June 2014 (UTC)
 * Yes . The largest value is 462, or possibly one larger value, it seems. —Quondum 16:54, 1 June 2014 (UTC)
 * Interesting!! If we could find one more term, that would disprove the GRH. (That's a big IF).  Bubba73 You talkin' to me? 18:03, 1 June 2014 (UTC)
 * Thank you, I did notice that the ones that I could find were largely square free, but hadn't put that together with the idea of a proof.Naraht (talk) 01:14, 2 June 2014 (UTC)
 * The proof given is seems to claim only that there are only two non-squarefree numbers in the sequence. It does not seem to say anything about squarefree numbers. I find it interesting (and not obvious) that the numbers are necessarily of the form p−1, p prime. —Quondum 02:47, 2 June 2014 (UTC)
 * I find the comment in the sequence description "(probably the list is complete)" odd. It can be easily verified that there are no other terms below 462. So the list is incomplete only if GRH is false - are they implying this is probably the case? -- Meni Rosenfeld (talk) 12:40, 2 June 2014 (UTC)
 * I don't follow you. What is known in that if the GRH is true, the list is complete, and if it is false, it may or may not have one more member (which would be greater than 1011). Since the GRH is considered "probably" true, the list is "probably" complete. (It can easily be computationally verified that there are no more elements up to a much larger number; I did so up to 107 before discovering the OEIS sequence by searching on the sequence that I found.) —Quondum 14:01, 2 June 2014 (UTC)
 * It's simple, apparently I'm blind. I thought it said "incomplete" which of course reverses the meaning. It makes sense now. -- Meni Rosenfeld (talk) 08:52, 3 June 2014 (UTC)

Infinity
How do you KNOW that numbers are infinite ? What is the largest number science has discovered to date ? — Preceding unsigned comment added by Ankoosh97 (talk • contribs) 10:37, 1 June 2014 (UTC)


 * Have you read infinity? Do that and come back. Plasmic Physics (talk) 12:45, 1 June 2014 (UTC)


 * It depends on what you mean by numbers. If you think of integers or reals, then none of them are infinite. It just exists infinitely many of them. YohanN7 (talk) 17:11, 1 June 2014 (UTC)
 * These infinities can be quantified by another type of number, the infinite cardinal numbers.


 * The largest number used in a documented mathematical proof is believed to be Graham's number.  → Michael J Ⓣ Ⓒ Ⓜ 06:30, 4 June 2014 (UTC)


 * Think about it in terms of the integers (whole numbers): suppose there were only a finite number of them... Then one of those would have to be the biggest (we could arrange them all in a list in order of size, and take the one at the end)... But then we could just take that number and add 1 to it. Now we've got a new whole number, and it isn't in our earlier list. That is, every time we suppose there is just a finite number of whole numbers, we end up finding another whole number that's not in our "finite" list. But this is crazy. Since we get a crazy result, we must have made a mistake earlier. So we must have been mistaken when we supposed there were only a finite number of whole numbers. So there isn't a finite number of whole numbers, so (by definition!) there must be an infinite number of them. RomanSpa (talk) 07:19, 6 June 2014 (UTC)

mathematics
quadratic equations problem — Preceding unsigned comment added by 123.238.158.211 (talk) 12:18, 1 June 2014 (UTC)


 * I'm not sure, but I suspect we'll need just a teensy bit more information. --   Jack of Oz   [pleasantries]  13:15, 1 June 2014 (UTC)


 * 42. Plasmic Physics (talk) 13:53, 1 June 2014 (UTC)


 * Are you looking for how to solve quadratic equations? You may have been introduced to a method where you "factorized" the expression, leaving you with a pair of brackets. You may have been introduced to a method called Completing the square. You may have been introduced to a method using the quadratic formula. If you can be more specific about what you remember, and what you are trying to do, we can help. Otherwise, BBC Bitesize might help you get started. 86.146.28.105 (talk) 19:18, 1 June 2014 (UTC)

Negative numbers
Are negative numbers "smaller" than positive numbers? Could a negative number ever be bigger than a positive number? Diechaddie (talk) 13:01, 1 June 2014 (UTC)


 * I assume you refer to magnitude when you say "smaller". If that is the case, then there is no difference. Plasmic Physics (talk) 13:16, 1 June 2014 (UTC)


 * Size can mean different things. It could mean the magnitude of a quantity (so the absolute value) in which case, yes there are negative numbers that are bigger than positive numbers.  For instance, &minus;10 is larger in magnitude than +2.  On the other hand "size" might refer to the natural ordering on the real numbers.  Every negative number is less than every positive number.  I would argue that the former notion of "size", "bigger", and "smaller" is the one that more commonly is used in conversation (for instance, we talk about "bigger debt" meaning a more negative balance).  Although obviously it depends on the context.  Sławomir Biały  (talk) 13:50, 1 June 2014 (UTC)


 * It depends on how you use them. For example, if you use positive to mean how much money you owe the bank, then negative numbers are what they owe you.  In that case, -$10 means they owe more than you would at +$2.


 * If, on the other hand, it's something that can't reverse direction, like elevation above sea level, you could think of +2 (2 meters above sea level) as higher than -10 (10 meters below sea level). However, -10 is farther from sea level, so it's still "bigger" in that sense.


 * There are even some strange cases where all negative values are bigger than all positives, such as stellar magnitudes, where the negative values are brighter. (I see no real reason why they had to do it this way, they just did.) StuRat (talk) 17:44, 1 June 2014 (UTC)


 * Another interesting example of all negative values being bigger than all positives is negative temperature. -- Meni Rosenfeld (talk) 20:25, 1 June 2014 (UTC)
 * Stellar magnitudes are actually rather simple. Historically, the brightest ones were of the first magnitude (like saying they were "first class"), and the next brightest were of the second magnitude (like "second class"), etc.  Now that we use the absolute magnitude scale, we have to redefine many stars' magnitudes: some of them are brighter than first magnitude, so sometimes we have to go into decimals or even negative numbers for them.  Nyttend (talk) 21:41, 1 June 2014 (UTC)


 * No, you're conflating two things. The apparent magnitude of Sirius A (as seen here) is -1.47; its absolute magnitude (apparent magnitude at 10 parsecs) is +1.42. —Tamfang (talk) 07:16, 2 June 2014 (UTC)
 * See the "History" section of Apparent magnitude. Historically, all stars (aside from the Sun) were placed into a six-magnitude scheme, with no decimals or negatives or anything like that.  Nyttend (talk) 02:41, 3 June 2014 (UTC)
 * I don't see where Apparent magnitude supports the idea that adoption of the modern logarithmic scheme was a consequence of the invention of absolute magnitude. I see that the first successful measurement of stellar parallax (on which absolute magnitude depends) was made in 1838, but how many of them were known in 1856 when Pogson proposed the logarithms? —Tamfang (talk) 09:46, 3 June 2014 (UTC)