Problems & Puzzles:
Puzzles
Puzzle 201.
The arithmetic function A(n)
The following puzzle was sent by
Luis Flavio Soares Nunes,
from Brazil.
Let n = p_{1}^{a}^{1}
^{. }...^{.} p_{r}^{a}^{r}
be the canonical representation of n.
We define A(n) = r ^{.}
max(a_{1}, ..., a_{r}).
This function has a geometrical propriety. In fact, if
we represent the prime factors of n in a rectangle, with the distinct p_{i}‘s
in its base, and each column with a_{i} primes p_{i} ,
then A(n) represents the minimum area of a rectangle that comports this
representation of n.
As example, let n = 2^{3} ^{.} 5^{2}
^{. }7^{6} ^{.} 11^{4}. Then, A(n) = 4
^{.} max(3, 2, 6, 4) = 4 ^{.} 6 = 24.
Or, we can draw this rectangle:
. 
. 
7 
. 
. 
. 
7 
. 
. 
. 
7 
11 
2 
. 
7 
11 
2 
5 
7 
11 
2 
5 
7 
11 
And its area is A(n) = 4 ^{.} 6 = 24.
We also
define an ordered conjunct
R_{k} = {n Є
N : A(n)=k}. We can see, by the above
example, that 2^{3} ^{.} 5^{2} ^{. }7^{6}
^{.} 11^{4} is in R_{24}.
Questions:
 For any k, what are the first
elements of R_{k }?
If k=1, its easy to describe R_{1}, simply
because it is the ordered set of all primes,
R_{1} = {2, 3, 5, 7, 11, ...}. However, if
k>1, it is difficult to enumerate R_{k}. We can prove that if k
is prime, R_{k} = {2^{k}, ...}. But, what is the next
element ? Can we express it ?
For example, by calculation, we discover that R_{2}
= {2^{2}, 2 ^{.} 3, 3^{2}, 2 ^{.} 5, 2
^{.} 7, 3 ^{.} 5, ...}. But, is
there a simpler manner of doing it ?
 Given k and M, how many elements
smaller than M are in R_{k}?
 Is there, in every set R_{k}
, 2 consecutive integers ?
For example, in R_{1} are 2 and 3. In R_{2}
are 14=2 ^{.} 7 and 15=3 ^{.} 5. R_{3} contains
2197=13^{3} and 2198=2 ^{.} 7 ^{.} 157, etc.
Does it holds in every set R_{k} ?
Solution:
Phil Carmody and Joseph L.
Pe sent interesting contributions to this puzzle:
Phil wrote:
1) If k is prime, the only elements of R_k are ones
of the form p^k and kalmostprimes. The first element will always be 2^k.
The second element will either be 3^k or nthprime(k)#
e.g.
k=2 either 9 or 6 => 6, i.e. nthprime(k)#
k=3 either 27 or 30 => 27 i.e. 3^k
k=4 either 81 or 210 => 81, and so on, 3^k
If k is a composite, then it can be split many ways,
and the smallest element for each split will be the only one that can
possibly be 2nd place, rather than 3^k. This smallest element will be the
almost empty rectangle consisting of a tower of twos, and a single
instance of each of the next smallest primes. i.e. 2^d.nthprime(k/d)#/2
for dk More than that, in this expression you tend to want the largest
number of powers of two, and certainly you never want d<(k/d). So the
number of values you want to look at is quite small, and it you should be
able to do it exhaustively almost instantly.
2) I can't see anything better than brute forcing
it. It looks like it's nonP in k, but bounded above by something linear
in M.
3) Given HeathBrown's results on who sigma and tau
can behave, I'd have to guess that every set will contain 2 consecutive
numbers. I reckon it can always be done with 1*k (flat) rectangles too
(i.e. kalmost primes).
Joseph wrote:
Recall that if n = p_1^a_1 * ... * p_r^a_r is the
canonical prime factorization of n, then A(n) is defined as A(n) = r *
max{a_1, ..., a_r}. The set R_k is defined as the set of natural numbers n
satisfying A(n) = k. Let R_k(1) < R_k(2) < .... be the elements of R_k in
increasing order.
Question 1. R_k(1), the first element of R_k, can be
characterized as follows. As already noted in your article, R_k(1) = 2^k
if k is prime. If k is not prime, then let d be a divisor of k and d' =
k/d. Consider the product P(d) = 2^d' * 3 * 5 * ... * p(d), where p(d) is
the dth prime. Then R_k(1) is the smallest value taken by P(d), that is:
R_k(1) = min_{d  k} 2^(k/d) * 3 * 5 * ... * p(d). More compactly, R_k(1)
= min_{d  k} 2^(k/d  1) * d#, where d# stands for d primorial, defined
as the product of the first d primes.
My characterization of R_k(2), the second element of
R_k, is not as neat, and depends upon having previously computed R_k(1).
If k = 2, then R_k(2) = 6; and if k is an odd prime, then R_k(2) = 3^k.
Now, if k is not prime, then let d be a divisor of k and d' = k/d.
Consider the product P(d) = 2^d' * q_2 * q_3 * ... * q_d, where q_2 < q_3
< ... < q_d is any sequence of d1 primes for which R_k(1) < P(d) < 3^k.
(3^k is probably not the tightest bound possible here.) Then R_k(2) is the
smallest value taken by P(d), that is: R_k(2) = min_{d  k, R_k(1) < P(d)
< 3^k} P(d).
Here is a table of the first and second elements of
R_k to illustrate the above expressions.
k first element of R_k second element of R_k
1 2 3^1
2 2^2 2^1 * 3^1
3 2^3 3^3
4 2^2 * 3^1 2^4
5 2^5 3^5
6 2^3 * 3^1 2^3 * 5^1
7 2^7 3^7
8 2^4 * 3^1 2^4 * 5^1
9 2^3 * 3^1 * 5^1 2^3 * 3^1 * 5^1
10 2^4 * 3^1 2^5 * 5^1
11 2^11 3^11
12 2^6 * 3^1 2^4 * 3^1 * 5^1
13 2^13 3^13
14 2^7 * 3^1 2^7 * 5^1
Ok, these "formulas" aren't really very practical.
To enumerate R_k, it is much easier to use an inductive procedure, speeded
up by a sieve method and other heuristics. The most obvious way to
enumerate R_k:
Base step. To compute R_k(1), compute A(2), A(3),
.... until the first t is found such that such that A(t) = k. Then R_k(1)
= t. Clearly, such t exists because A(2^k) = k, so that R_k(1) <= 2^k. To
speed up the search, it is easily seen that R_k(1) must be a multiple of 4
for k > 1, so that only the 2^(k2) multiples of 4 that are <= 2^k need to
be considered.
Inductive step. Suppose R_k(1) < R_k(2) < ... <
R_k(s) have been found; then R_k(s+1) has to be determined. Compute
A(R_k(s) + 1), A(R_k(s) + 2), .... until the first t among R_k(s) + 1,
R_k(s) + 2, .... is found such that A(t) = k. Such t exists because if
R_k(s) = p_1^a_1 * ... * p_r^a_r is the canonical prime factorization of
R_k(s), then the number U = p_1^a_1 * ... * (p')^a_r, obtained by
replacing p_r by the next prime p', is > R_k(s) and A(U) = k, so that
R_k(s+1) <= U. Put R_k(s+1) = t.
This simple enumeration can be improved by
incorporating a sieve method (as in the sieve of Eratosthenes). Start with
the list {2, 3, ....}. Proceed as above, computing A(2), A(3), .... If
A(i) = k, encircle the integer i. If A(i) is not equal to k, then
underline i. In addition, if A(i) > k, then A(I) >= A(i) > k for any
multiple I of i; hence, underline every ith number in the list
(corresponding to a multiple of i). Compute A(i) only for i that has not
yet been encircled or underlined. The (ordered) list of encircled numbers
enumerates the elements of R_k.
Here is a Mathematica program (implementing just the
"obvious" method) to enumerate R_4 and print any pairs of
consecutive elements:
A[n_] := Module[{pf}, pf = Transpose[FactorInteger[n]];
Length[pf[[1]]]*Max[pf[[2]]]];
R[k_] := Module[{l, ub, i, lastval},
l = {};
ub = 10^4;
lastval = 0;
For[i = 2, i <= ub, i++,
If[A[i] == k,
l = Append[l, i];
If[i == lastval + 1,
Print[ToString[(i  1)] <> " and " <> ToString[i] <>
" are consecutive elements"]];
lastval = i
]];
l];
R[4]
Question 3. I have verified (using the program
above) that R_1, ..., R_6 have consecutive elements, but there are no
consecutive elements < 10^8 in R_7. I suspect the answer to this question
is "no", but I have only empirical evidence.
I asked to Phil what was his
opinion on the Pe's comment to question 3, a kind of divergent of
his own comment based on the HeathBrown's work. This is what he
answered (are you ready for it?)
The case R_p with p prime does seriously restrict
the number of ways of finding values. R_k is contains 7th powers, and
products of 7 primes, doesn't it?
19203908986158 = 2*3*13*281*337*1289*2017 precedes
19203908986159 = 79^7
The following are also consecutive
powers/7almostprimes
prime [factorization of 7almostprime] prime^7
power > 7almostprime:
191 [2, 5, 19, 127, 197, 10627, 183569]
9273284218074431
223 [2, 3, 29, 37, 491, 1709, 5076443] 27424204663190047
359 [2, 179, 211, 449, 1303, 4019, 4327] 768530557342240919
1283 [2, 29, 631, 641, 3739, 24781, 2632673] 5722510452670311609227
(after these a long list of other solutions)
power < 7almostprime:
457 [2, 29, 43, 229, 421, 7309, 2368871]
4163067000501310393
709 [2, 5, 29, 71, 6287, 534857, 1300727] 90058279596528053869
761 [2, 3, 71, 113, 127, 105883, 228336109] 147806181513219494921
821 [2, 3, 29, 137, 197, 631, 84847040587] 251421318647928806141
(after these a long list of other solutions)
These are just the ones <10^35. I have no reason to
suspect that they thin out so much that they ever run out. I've not even
thought of looking at consecutive 7almostprimes, these would be _much
much_ more common than ones that are adjacent to prime powers, such as
those found above.
The work of HeathBrown _proves_ that you will find
consecutive kalmost primes for every k. So my gut feel isn't required.
Every set will have consecutive terms. Full stop.
So restricting ourselves to the harder problem of
primepowers 'towers' next to flatter arrangements...
R_8 is in some ways harder (a sparse form, and one
that has a massively factorable neighbor to the power). However, there are
several 'tower blocks' that work [1,1,1,1,1,1,1,1], [2,1,1,1], [2,2,1,1],
[2,2,2,1], [2,2,2,2], [4,1], [4,2], [4,3], [4,4], which makes it easier.
Here are the solutions up to 10^40:
prime power < 8almostprime:
131 [2, 17, 7841, 19136877329] 86730203469006241
709 [2, 17, 3089, 35762192350095041] 63851320233938390193121
1231 [2, 17, 72648689, 125576624052401] 5273080767529718372423041
(after these a long list of other solutions)
prime power > 8almostprime:None, too make
algebraic factors of p^81
R_9 has swings in its favor and almost nothing
against its favor
347 [2, 7, 13, 19, 73, 173, 1297, 1327, 970421527]
72939918399605131977467
947 [2, 7, 11, 19, 43, 109, 277, 463, 348272974918243]
612561660047653251978446867
(after these a long list of other solutions)
R_10 also has some results, a few more that R_8, but
looking similarly onesided due to the algebraic factorization of p^101.
17 [2, 5, 29, 21881, 63541] 2015993900449
37 [2, 5, 61, 137, 11507920001] 4808584372417849
R_11 refused to give me a result, but searching is
harder as you're looking at 50+digit numbers. It would be a drag to try to
optimize the code to earlyabort if it can't find enough small factors
quickly, as I'm using pari/gp, and I'm not a particularly good programmer
in that language. (It's 2 lines of code to find the above results in
pari/gp!). So I hope that's put the R_7 problem to bed. Alas it's awakened
a R_11 problem instead :)...
No it hasn't  this just in:
6343 [2, 3, 7, 23, 151, 1409, 1607, 311323, 405989,
502608943, 31871697617] 668716170795985943898848282166258251528407
Uf!!!... Thanks Phil.
***
Frank Rubin sent the following
comment (20/11/2002) regarding the same issue of the question 3:
I think that every R_k contains not
just 2 consecutive integers, but longer sequences of n consecutive
integers as well. For example, R_5 contains 1042404, 1042405 and 1042406.
Sudipta Das
and I found similar sequences for other values of k and n.
***
Later Rubin added a more
complete table of results and comments:
I have now
searched all of the integers through 2,100,000,000 for consecutive
integers having the same number of prime factors. I made two separate
runs, one for distinct prime factors, like the puzzle on my website, and
total prime factors, like the puzzle on your website.
The results are somewhat different from what I had expected. There do
not seem to be any sequences longer than 8 consecutive numbers having 2
distinct prime factors. The reason for this is that, just as the
primes thin out as you get larger, the numbers that can be expressed as
the product of 2 primes thin out. I think this will eventually happen
with 3 primes, 4 primes, and so forth. However, it will occur at points
much greater than 2.1 billion.
With total prime factors the length of the sequences is limited by the
number of factors. With n factors there cannot be more than 2^n1
consecutive integers. This is because any 2^n consecutive integers will
contain a multiple of 2^n, which must have more than n factors. (The
first multiple, namely 1x2^n cannot be part of a sequence of consecutive
integers with n factors. 2^n is the first integer with n factors, so
2^n1 has fewer than n factors. The next integer with n factors will be
3x2^(n1) so 2^n+1 also must have fewer than n factors.)
My belief is that for 4 total factors there will be sequences of 15
consecutive integers. It is less clear if this will hold true for
larger numbers of factors, for example that there will actually be
sequences of 31 consecutive integers having 5 total factors.
I have attached two tables showing the smallest integer that begins a
sequence of n consecutive integers with k factors.
Frank Rubin Dec. 7, 2002
* Distinct prime factors
Run k Number of factors
Len 2 3 4 5 6 6 7 8 9
        
1: 6 30 210 2310 30030 510510 9699690 223092870
2: 14 230 7314 254540 11243154 965009045 0 0
3: 20 644 37960 1042404 323567034 0 0 0
4: 33 1308 134043 21871365 0 0 0
5: 54 2664 357642 129963314 0 0 0
6: 91 6850 1217250 830692265 0 0 0 0
7: 141 10280 1217250 0 0 0 0 0
8: 141 39693 14273478 0 0 0 0 0
9: 0 44360 44939642 0 0 0 0 0 0
10: 0 48919 76067298 0 0 0 0 0
11: 0 218972 163459742 0 0 0 0 0
12: 0 526095 547163235 0 0 0 0 0
13: 0 526095 2081479430 0 0 0 0 0
14: 0 526095 0 0 0 0 0 0
15: 0 17233173 0 0 0 0 0 0
16: 0 127890362 0 0 0 0 0 0
* Total prime factors
Run k Number of factors
Len 2 3 4 5 6 7 8 9 10
         
1: 4 8 16 32 64 128 256 512 1024
2: 9 27 135 944 5264 29888 50624 203391 3290624
3: 33 170 1274 15470 33614 3145310 40909374 668363967 0
4: 0 602 4023 57967 8706123 296299374 0 0 0
5: 0 602 12122 632148 101905622 0 0 0 0
6: 0 2522 204323 14845324 0 0 0 0 0
7: 0 211673 355923 69921004 0 0 0 0 0
8: 0 0 3405122 888781058 0 0 0 0 0
9: 0 0 3405122 0 0 0 0 0 0
10: 0 0 49799889 0 0 0 0 0 0
11: 0 0 202536181 0 0 0 0 0 0
***
Trial Brian sent a note (10/12/2002)
reminding that he solved the
CYF No. 8, of
the Shyam Sunder Gupta's site, related to
the Rubin's belief "My belief is that for 4 total factors there will be sequences of 15
consecutive integers"
***
