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February 9

How to find a multiplier so that the resulting number is a small perfect square…

Simple question, let’s take a semiprime $i$ .
I want to find a number $x$ such as x×i is perfect square (the square root is an integer) but the resulting square is small than $i^{2}$ 2A01:E0A:401:A7C0:9407:59CB:EFF:777C (talk) 20:36, 9 February 2025 (UTC)[reply]

Is this a joke? If a square has a prime factor

p

, it is divisible by

p^{2}.

If it has distinct prime factors

p_{1},p_{2},...,p_{n}

, it is divisible by the lcm of

p_{1}^{2},p_{2}^{2},...,p_{n}^{2}

, which, since all these are co-prime, equals

p_{1}^{2}p_{2}^{2}\cdots p_{n}^{2}.

So a square that is a multiple of

i=p_{1}p_{2}

is a multiple of

i^{2}.

‑‑Lambiam 21:35, 9 February 2025 (UTC)[reply]

IP, I don't know if you're still looking for an answer, but this is only possible for some numbers i. I'll start with a few examples before trying to discuss the general case.

First example: suppose i = 12. The prime factorization is 12 = 2x2x3 = $(2^{2})(3)$ . Notice that 2 is raised to an even power, but 3 is raised to an odd power. Since you're looking for x such that (x)(2^2)(3) is a perfect square, each prime in the the prime factorization of (x)(i) must be raised to an even power. In this case, if x=3, then (x)(i) = $(2^{2})(3^{2})$ = 36, which is a perfect square that's smaller than $i^{2}=12^{2}$ = 144.
Second example: suppose i = 216. The prime factorization is 216 = 2x2x2x3x3x3 = $(2^{3})(3^{3})$ . Notice that both 2 and 3 are raised to an odd power this time. Since you're looking for x such that (x) $(2^{3})(3^{3})$ is a perfect square, one way for that to occur is if you let x=(2)(3), in which case (x)(i) = $(2^{4})(3^{4})$ , which is a perfect square that's smaller than $i^{2}=((2^{3})(3^{3}))^{2}=(2^{6})(3^{6})$ . Another way for this to occur is if you let x = $(2^{3})(3)$ , in which case (x)(i) = $(2^{6})(3^{4})$ , which is again smaller than $i^{2}=(2^{6})(3^{6})$ , and similarly if you let x = $(2)(3^{3})$ .
Third example: suppose i = 100. The prime factorization is 100 = 2x2x5x5 = $(2^{2})(5^{2})$ . Notice that both 2 and 5 are raised to an even power this time. Since you're looking for x such that (x) $(2^{2})(5^{2})$ is a perfect square, one way for that to occur is if you let x= $2^{2}$ , in which case (x)(i) = $(2^{4})(5^{2})$ , which is a perfect square that's smaller than $i^{2}=((2^{2})(5^{2}))^{2}=(2^{4})(5^{4})$ . Another way for this to occur is if you let x = $(5^{2})$ , in which case (x)(i) = $(2^{2})(5^{4})$ , which is again smaller than $i^{2}=(2^{4})(5^{4})$ , and yet another way is if you let x = $(3^{2})$ , in which case (x)(i) = $(2^{2})(3^{2})(5^{2})$ , which is again smaller than $i^{2}=(2^{4})(5^{4})$ .
Fourth example: suppose i = 30. The prime factorization is 30 = 2x3x5, where each prime is raised to an odd number, and all of the powers are equal to 1. The problem in this example is that the smallest x such that (x)(i) is a perfect square is x = (2)(3)(5). But in this case (x)(i) = $i^{2}$ , so there is no x that satisfies your condition of (x)(i) being smaller than $i^{2}$ .

Hopefully you can now move to the general case, where the prime factorization is i =

p 1 a 1 p 2 a 2 ...p n a n

. If all of the

a_{n}

are equal to 1, there is no x that satisfies your condition. If one or more of the

a_{n}

are greater than 1, then you choose x such that all of the primes in the the prime factorization of (x)(i) will be raised to an even power. The smallest x will be the product of the primes in the prime factorization of i that are raised to an odd power (e.g., if i =

p 1 a 1 p 2 a 2 ...p n a n

, and

a_{1},a_{3}

, and

a_{4}

are all of the odd powers, then the smallest x is x =

(a_{1})(a_{3})(a_{4})

, and you'll also be able to find some other values for x by multiplying that smallest value by a small perfect square). FactOrOpinion (talk) 20:59, 16 February 2025 (UTC)[reply]

Note, though, that this implies that

i

is not a semiprime as supposed in the question. ‑‑Lambiam 04:14, 17 February 2025 (UTC)[reply]

February 10

Why this equation has no solutions ?

Take $SA$ , a semiprime, why is there no solution to this equation

(25² + x×SA)÷(67×y) == (z×SA)²

I’m meaning finding integers x and y and z such as the equation is true and where z≠0 ? 2A01:E0A:401:A7C0:E9E3:F9ED:9B83:6DB8 (talk) 14:36, 10 February 2025 (UTC)[reply]

I'm not certain what SA is supposed to be but if it is 1 and z is 1 then it is quite easy to get a solution of x=45 and y=10. NadVolum (talk) 15:24, 10 February 2025 (UTC)[reply]

Sorry, edited.

SA

is semiprime. 2A01:E0A:401:A7C0:C965:8D1D:CEAE:ABA0 (talk) 17:23, 10 February 2025 (UTC)[reply]

Multiplying both sides by

67y

and taking both sides modulo

SA

, we see that

25^{2}=5^{4}

would have to be congruent to

0\!\!{\pmod {SA}}

. In other words,

5^{4}

would have to be a multiple of a semiprime, which is impossible since it only has one prime factor. GalacticShoe (talk) 17:43, 10 February 2025 (UTC)[reply]

and if 25 was replaced by a square which is a multiple of the semiprime, would it works ? In reality 25 can be any perfect square, it doesn’t have to be 25. 2A01:E0A:401:A7C0:C965:8D1D:CEAE:ABA0 (talk) 02:59, 11 February 2025 (UTC)[reply]

The equation

{\frac {((m\times {\text{SA}})^{2})^{2}+x\times {\text{SA}}}{67\times y}}=(z\times {\text{SA}})^{2}

is a linear equation in

x.

It has a unique solution for any choice of non-zero

{\text{SA}},

m,

non-zero

y,

and

z.

If all of these are integers, then so is

x.

‑‑Lambiam 06:38, 11 February 2025 (UTC)[reply]

With

m

being a perfect square ? If yes, how to compute it ? 2A01:E0A:401:A7C0:C965:8D1D:CEAE:ABA0 (talk) 13:50, 11 February 2025 (UTC)[reply]

You wrote, "if 25 was replaced by a square which is a multiple of the semiprime". Well,

(m\times {\text{SA}})^{2}

is a square that is a multiple of

{\text{SA}}.

You don't need to compute

m

;

m

can be set to any value that you wish, including squares, so you can take

m=100,

or

m=1,

or even

m=0.

Aren't you overdoing the squaring? If

m=a^{2}

and you expand the brackets, you end up with

{{a^{2}}^{2}}^{2}{{\text{SA}}^{2}}^{2}=a^{8}{\text{SA}}^{4}.

‑‑Lambiam 21:30, 11 February 2025 (UTC)[reply]

Yes, that’s correct, with the ²

m

is a perfect square by construction. But how to find a value of

m

such as a solution of

x

and

y

and

z

exists ? 2A01:E0A:401:A7C0:88B1:8A9D:F4E0:D37A (talk) 07:24, 13 February 2025 (UTC)[reply]

Take

m=1.

‑‑Lambiam 09:27, 13 February 2025 (UTC)[reply]

February 14

Could eversion be a functor?

After reading Sphere eversion, which I didn't understand(not the article's fault), I wonder if there is a class of objects including the 3 particular spheres, and maybe tori that can be everted and form a category, in which eversion would be a functor from that category to itself.Rich (talk) 16:16, 14 February 2025 (UTC)[reply]

Presumably, in a categorical treatment, the objects would be self-intersection-free immersions of bounded and connected 2-dimensional smooth orientable manifolds in three-dimensional space. The morphisms would be, presumably, regular homotopies. Some of these morphisms preserve orientation; the others are "disorienting" – they flip the orientation around. Some morphisms are endomorphisms: their domain and codomain are the same object. Eversions are then the disorienting endomorphisms, a subclass of the morphisms. This subclass is not particularly interesting, categorically speaking, since the composition of two eversions is not an eversion. We can form a subcategory whose objects are the evertible objects, but we cannot restrict the morphisms to just the eversions, because the identity morphisms would thereby be excluded.

An endofunctor of this category should map morphisms to morphisms; eversions, being themselves morphisms, live at a lower categorical level. I see no obvious way of defining another notion of morphism between evertible objects in which an endofunctor might correspond to the notion of eversion. ‑‑Lambiam 06:05, 15 February 2025 (UTC)[reply]

February 17

Infinitely many pairs of consecutive primes with given last digits

For which pairs $(d_{1},d_{2})\in \{1,3,7,9\}\times \{1,3,7,9\}$ is it known that there are infinitely many primes ending with the digit d₁ for which the next prime ends with the digit d₂? Note that there are 16 possible pairs to consider.

If one arranges the 16 sets of pairs of consecutive primes into a $4\times 4$ -grid, then it is easy to see that at least one set in each row and one in each column must be infinite (by Dirichlet's theorem on arithmetic progressions). Perhaps, all 16 sets might turn out to be infinite.

More generally, the problem could be generalized to bases other than 10. In this case, for base b, there are φ(b)² possible combinations for the mod-b remainders (or equivalently, the last digits in base b) of pairs of consecutive primes greater than the largest prime dividing b, where φ(b) is Euler's totient function. Another possible generalization would be to consider triplets or n-tuples of consecutive primes. In this case, there are φ(b)ⁿ possible combinations for the mod-b remainders (or equivalently, the last digits in base b) of n-tuples of consecutive primes greater than the largest prime dividing b. The corresponding conjecture would then be that all of the resulting φ(b)ⁿ sets of n-tuples of consecutive primes are infinite. GTrang (talk) 05:27, 17 February 2025 (UTC)[reply]

For what it's worth, solving b=5 would also solve b=10 since all primes after 2 are odd. It seems reasonable to expect that 16 entries in the grid to be infinite; primes seem to behave randomly in every way they might be expected to behave randomly, but proving the corresponding conjectures usually proves elusive. Dirichlet's theorem is an example of this, since even though the behavior in question is very simple, the proof is very complex. The case b=2 is trivial, but other than that I think a more realistic question is if there has been any research or partial results on the question. --RDBury (talk) 08:44, 17 February 2025 (UTC)[reply]

PS. For b=4 it's not hard to see that Dirichlet's theorem implies the (1, 3) and (3, 1) entries must be infinite. Otherwise there would be a point after which the last digits would be all 1 or all 3. It's not hard to find data on-line, for example here for b=10 and here for b=4. I imported the b=4 data into a spreadsheet and found the entries in the grid for the first 10000 primes. The results were (1, 1):2053, (1, 3):2930, (3, 1):2931, (3, 3):2084. As you can see the (1, 3) and (3, 1) entries are significantly larger than the (1, 1) and (3, 3), and from what I can tell this doesn't seem to a transient effect; when you restrict the range to 9000 to 10000 the imbalance doesn't go away. I have no explanation for this. A similar experiment with the entries in the b=10 grid also appear to be significantly imbalanced. --RDBury (talk) 09:30, 17 February 2025 (UTC)[reply]

A (1, 3) pair can be twins (e.g. 11 – 13 or 4241 – 4243) and a (3, 1) pair can be just 8 apart (e.g. 683 – 691 or 8753 – 8581), whereas (1, 1) and (3, 3) require a prime gap that is at least 10. In the beginning of the sequence of prime gaps (OEIS A001223) larger gaps are relatively rare. Around 10000 the average gap is about log(10000) ≈ 9.2, but the imbalance may eventually disappear when the average gap is substantially larger than 10. ‑‑Lambiam 11:02, 17 February 2025 (UTC)[reply]

Right. I was thinking b = 4, so to me a (1, 3) pair would be 17, 19. If the "probably" of a number being prime is p, then the "probability" of a (1, 3) or (3, 1) pair is p + (1-p)²p + (1-p)⁴p ... = p/(1-(1-p)²). = p/(2p-p²) = 1/(2-p). Similarly, the "probability" of a (1, 1) or (3, 3) pair is (1-p)/(2-p). These do converge to 1/2 as p→0, but seeing as p is roughly inversely proportional to the number of digits (by the prime number theorem), the convergence will be very slow and the difference will still be noticeable even going up to the 10000th prime (6 digits). It would be interesting to compare different ranges of primes to see if the amount of imbalance actually matches what the prime number theorem predicts, but I'm satisfied for now. --RDBury (talk) 15:23, 17 February 2025 (UTC)[reply]

Leibniz rule for antiderivatives

The general Leibniz rule is a formula for computing higher derivatives of products:

(fg)^{(n)}=\sum _{k=0}^{n}{n \choose k}f^{(n-k)}g^{(k)}.

This is analogous to the binomial theorem, and in fact, as pointed out in the article, the binomial theorem can be proven from the special case f=exp(ax), g=exp(bx). But the binomial theorem can be generalized to negative n using the binomial series. So it seems reasonable (at least to me) to generalize the Leibniz rule to find a formula for (fg)⁽ⁿ⁾ when n is negative, where f^(-k) is interpreted to mean a kth antiderivative of f. An antiderivative is only defined up to a constant, but this can resolved by stipulating, say, f^(-k)(0) = 0. For example, we have:

(fg)^{(-1)}=f^{(-1)}g-f^{(-2)}g^{(1)}+f^{(-3)}g^{(2)}-f^{(-4)}g^{(3)}\dots

when the series converges; this is iterated integration by parts. (Note that if g is polynomial then the sum is finite and convergence is not an issue.) Similarly:

(fg)^{(-2)}=f^{(-2)}g-2f^{(-3)}g^{(1)}+3f^{(-4)}g^{(2)}-4f^{(-5)}g^{(3)}\dots

and more generally:

(fg)^{(-k)}=f^{(-k)}g-{k \choose 1}f^{(-k-1)}g^{(1)}+{k+1 \choose 2}f^{(-k-2)}g^{(2)}-{k+2 \choose 3}f^{(-k-3)}g^{(3)}\dots \ .

This seems relatively straightforward to prove, given sufficient hand-waving on convergence and arbitrary constants, and useful since I happened to need a formula for (xⁱ exp(x))^(-k), so it seems like this is the kind of thing that would be well known and documented. But my searches have not turned up anything; I get a lot of results for the Leibniz integral rule instead, and that's very different. So do these formulas look familiar to anyone? --RDBury (talk) 10:54, 17 February 2025 (UTC)[reply]