# Tag: Ramanujan

Yesterday, there was an interesting post by John Baez at the n-category cafe: The Riemann Hypothesis Says 5040 is the Last.

The 5040 in the title refers to the largest known counterexample to a bound for the sum-of-divisors function
$\sigma(n) = \sum_{d | n} d = n \sum_{d | n} \frac{1}{n}$

In 1983, the french mathematician Guy Robin proved that the Riemann hypothesis is equivalent to
$\frac{\sigma(n)}{n~log(log(n))} < e^{\gamma} = 1.78107...$ when $n > 5040$.

The other known counterexamples to this bound are the numbers 3,4,5,6,8,9,10,12,16,18,20,24,30,36,48,60,72,84,120,180,240,360,720,840,2520.

In Baez’ post there is a nice graph of this function made by Nicolas Tessore, with 5040 indicated with a grey line towards the right and the other counterexamples jumping over the bound 1.78107…

Robin’s theorem has a remarkable history, starting in 1915 with good old Ramanujan writing a part of this thesis on “highly composite numbers” (numbers divisible by high powers of primes).

His PhD. adviser Hardy liked his result but called them “in the backwaters of mathematics” and most of it was not published at the time of Ramanujan’s degree ceremony in 1916, due to paper shortage in WW1.

When Ramanujan’s paper “Highly Composite Numbers” was first published in 1988 in ‘The lost notebook and other unpublished papers’ it became clear that Ramanujan had already part of Robin’s theorem.

Ramanujan states that if the Riemann hypothesis is true, then for $n_0$ large enough we must have for all $n > n_0$ that
$\frac{\sigma(n)}{n~log(log(n))} < e^{\gamma} = 1.78107...$ When Jean-Louis Nicolas, Robin's PhD. adviser, read Ramanujan's lost notes he noticed that there was a sign error in Ramanujan's formula which prevented him from seeing Robin's theorem.

Nicolas: “Soon after discovering the hidden part, I read it and saw the difference between Ramanujan’s result and Robin’s one. Of course, I would have bet that the error was in Robin’s paper, but after recalculating it several times and asking Robin to check, it turned out that there was an error of sign in what Ramanujan had written.”

If you are interested in the full story, read the paper by Jean-Louis Nicolas and Jonathan Sondow: Ramanujan, Robin, Highly Composite Numbers, and the Riemann Hypothesis.

What’s the latest on Robin’s inequality? An arXiv-search for Robin’s inequality shows a flurry of activity.

For starters, it has been verified for all numbers smaller that $10^{10^{13}}$…

It has been verified, unconditionally, for certain classes of numbers:

• all odd integers $> 9$
• all numbers not divisible by a 25-th power of a prime

Rings a bell? Here’s another hint:

According to Xiaolong Wu in A better method than t-free for Robin’s hypothesis one can replace the condition of ‘not divisible by an N-th power of a prime’ by ‘not divisible by an N-th power of 2’.

Further, he claims to have an (as yet unpublished) argument that Robin’s inequality holds for all numbers not divisible by $2^{42}$.

So, where should we look for counterexamples to the Riemann hypothesis?

What about the orders of huge simple groups?

The order of the Monster group is too small to be a counterexample (yet, it is divisible by $2^{46}$).

(After-math of last week’s second year lecture on elliptic
curves.) We all know the story of
Ramanujan and the
taxicab, immortalized by
Hardy

I
remember once going to see him when he was lying ill at Putney. I had
ridden in taxicab no. 1729 and remarked that the number seemed to me
rather a dull one, and that I hoped it was not an unfavorable omen.
‘No,’ he replied, ‘it’s a very interesting number; it is the smallest
number expressible as a sum of two cubes in two different ways’.

When I was ten, I wanted to become an archeologist and
even today I can get pretty worked-up about historical facts. So, when I
was re-telling this story last week I just had to find out things like :
the type of taxicab and how numbers were displayed on them and, related
to this, exactly when and where did this happen, etc. etc. Half an hour
free-surfing further I know a bit more than I wanted. Let’s start with
the date of this taxicab-ride, even the year changes from source to
source, from 1917 in the dullness of 1729 (arguing that Hardy
could never have made this claim as 1729 is among other things the third
Carmichael Number, i.e., a pseudoprime relative to EVERY base) to ‘late
in WW-1’ here… Between 1917
and his return to India on march 13th 1919, Ramanujan was in and out a
number of hospitals and nursing homes. Here’s an attempt to summarize
these dates&places (based on the excellent paper Ramanujan’s Illness by D.A.B. Young). (may 1917 –
september 20th 1917) : Nursing Hostel, Thompson’s Lane in Cambridge.
(first 2 a 3 weeks of october 1917) : Mendip Hills Senatorium, near
Wells in Somerset. (november 1917) : Matlock House Senatorium at
Matlock in Derbyshire. (june 1918 – november 1918) : Fitzroy House, a
hospital in Fitzroy square in central London. (december 1918 – march
1919) : Colinette House, a private nursing home in Putney, south-west
London. So, “he was lying ill at Putney” must have meant that Ramanujan
was at Colinette House which was located 2, Colinette Road and a quick

shows that the
The British Society for the History of Mathematics Gazetteer
is correct in asserting that “The house is no longer used as a nursing
home and its name has vanished” as well as

It was in 1919
(possibly January), when Hardy made the famous visit in the taxicab
numbered 1729.

Hence, we are looking for a London-cab
early 1919. Fortunately, the London Vintage Taxi
Association
has a website including a
taxi history page.

At
the outbreak of the First World War there was just one make available to
Production of the Unic ceased for the duration as the company turned to
producing munitions. The majority of younger cabmen were called up to
fight and those that remained had to drive worn-out cabs. By 1918 these
remnant vehicles were sold at highly inflated prices, often beyond the
pockets of the returning servicemen, and the trade deteriorated.

and as the first post-war taxicab type was introduced in
1919 (which became known as the ‘Rolls-Royce of cabs’) more than likely
the taxicab Hardy took was a Unic

and the number
1729 was not a taxicab-number but part of its license plate. I still
dont know whether there actually was a 1729-taxicab around at the time,
but let us return to mathematics. Clearly, my purpose to re-tell the
story in class was to illustrate the use of addition on an elliptic
curve as a mean to construct more rational solutions to the equation
$x^3+y^3 = 1729$ starting from the Ramanujan-points (the two
solutions he was referring to) : P=(1,12) and Q=(9,10). Because the
symmetry between x and y, the (real part of) curve looks like

and if we take
0 to be the point at infinity corresponding to the asymptotic line, the
negative of a point is just reflexion along the main diagonal. The
geometric picture of addition of points on the curve is then summarized
in

and sure
enough we found the points
$P+Q=(\frac{453}{26},-\frac{397}{26})$ and
$(\frac{2472830}{187953},-\frac{1538423}{187953})$ and so on
by hand, but afterwards I had the nagging feeling that a lot more could
side remark : I learned of this example from the excellent book by Alf
Van der Poorten Notes on Fermat’s last theorem page 56-57. Alf acknowledges that he
borrowed this material from a lecture by Frits Beukers ‘Oefeningen rond
Fermat’ at the National Fermat Day in Utrecht, November 6th 1993.
Perhaps a more accurate reference might be the paper Taxicabs and sums of two cubes by Joseph
Silverman which appeared in the april 1993 issue of The American
Mathematical Monthly. The above drawings and some material to follow is
taken from that paper (which I didnt know last week). I could have
proved that the Ramanujan points (and their reflexions) are the ONLY
integer points on $x^3+y^3=1729$. In fact, Silverman gives a
nice argument that there can only be finitely many integer points on any
curve $x^3+y^3=A$ with $A \in \mathbb{Z}$ using the
decomposition $x^3+y^3=(x+y)(x^2-xy+y^2)$. So, take any
factorization A=B.C and let $B=x+y$ and
$C=x^2-xy+y^2$, then substituting $y=B-x$ in the
second one obtains that x must be an integer solution to the equation
$3x^2-3Bx+(B^2-C)=0$ Hence, any of the finite number of
factorizations of A gives at most two x-values (each giving one
y-value). Checking this for A=1729=7.13.19 one observes that the only
possibilities giving a square discriminant of the quadratic equation are
those where $B=13, C=133$ and $B=19, C=91$ leading
exactly to the Ramanujan points and their reflexions! Sure, I mentioned
in class the Mordell-Weil theorem stating that the group of rational
solutions of an elliptic curve is always finitely generated, but wouldnt
it be fun to determine the actual group in this example? Surely, someone
must have worked this out. Indeed, I did find a posting to
sci.math.numberthy by Robert L. Ward : (in fact, there is a nice page on
from clippings to this newsgroup)  From: rlward1@orion.ncsc.mil
 (Robert L. Ward) Subject: Re: the MW group of the taxicab cubic Date: 10 Nov 99 16:16:22 GMT Newsgroups: sci.math.numberthy I used MAGMA to answer this question. The minimal model of this curve is: Y^2 + Y = X^3 - 20178727, its discriminant is -3^9_7^4_13^4*19^4, and its conductor is 80714907 = 3^3_7^2_13^2*19^2. The transformation to get this equation from x^3 + y^3 = 1729 is X = 5187/(y+x), Y = (7780_y-7781_x)/(x+y),<br /> x = (7780-Y)/(3_X), y = (7781+Y)/(3_X). I issued the following commands: E := EllipticCurve([0,1,0,0,-20178727]);<br /> MW,h := MordellWeilGroup(E); P := h(MW.1); Q := h(MW.2); Then P turned out to be (273,409) and Q to be (399,6583), which correspond to the Ramanujan solutions of the original equations. Thus MAGMA asserts that generators of the Mordell-Weil group of the 
curve are indeed the Ramanujan points. Robert L. Ward  This
would be great! all rational solutions of the taxicab-curve are
constructed from the Ramanujan points by addition on the elliptic
curve. However, the lost archeologist in me wanted to check
this himself… Ive never done calculations with elliptic curves in
sage so now was a good time
to learn this. Sage assumes to input your elliptic curve in
Weirstrass-form but fortunately Roberts transformations above transfer
the taxicab-curve to Weierstrass form $y^2+y=x^3-20178727$
and sage accepts the array-input $(a_1,a_2,a_3,a_4,a_6)$ of an
elliptic curve defined by $y^2+a_1xy+a_3y = x^3+a_4x+a_6$ and
sage has some very powerful routines for elliptic curves, including John
Cremona’s
Mwrank
function to compute the rank of the Mordell-Weil group and it can also
determine the torsion part and generators of the Mordell-Weil group. So,
here we go (and it returned it all in some 5 minutes on my MacBook)
 sage: T=EllipticCurve([0,0,1,0,-20178727]) sage:
 T.torsion_subgroup() Trivial Abelian Group sage: T.rank() 2 sage: T.gens() [(273 : 409 : 1), (1729 : 71753 
1)]  The Mordell-Weil group of the taxicab-curve is isomorphic
to $\mathbb{Z} \oplus \mathbb{Z}$ and the only difference with
Robert Wards posting was that I found besides his generator
$P=(273,409)$ (corresponding to the Ramanujan point (9,10)) as
a second generator the point $Q=(1729,71753)$ (note again the
appearance of 1729…) corresponding to the rational solution $( -\frac{37}{3},\frac{46}{3})$ on the taxicab-curve. Clearly, there
are several sets of generators (in fact that’s what
$GL_2(\mathbb{Z})$ is all about) and as our first generators
were the same all I needed to see was that the point corresponding to
the second Ramanujan point (399,6583) was of the form $\pm Q + a P$ for some integer a. Points and their addition is also easy to do
with sage  sage: P=T([273,409]) sage: Q=T([1729,71753]) sage: -P-Q (399 : 6583 : 1)  and we see that the
second Ramanujan point is indeed of the required form!