a problem from The Strand

murshid_islam

I was reading Ramanujan’s biography (by Kanigel) and there was a mathematical problem in that book which was published in the Strand magazine during the First World War. This is the problem:

"I was talking the other day," said William Rogers to the other villagers gathered around the inn fire, "to a gentleman about the place called Louvain, what the Germans have burnt down. He said he knowed it well – used to visit a Belgian friend there. He said the house of his friend was in a long street, numbered on this side one, two, three, and so on, and that all the numbers on one side of him added up exactly the same as all the numbers on the other side of him. Funny thing that! He said he knew there was more than fifty houses on that side of the street, but not so many as five hundred. I made mention of the matter to our parson, and he took a pencil and worked out the number of the house where the Belgian lived I don’t know how he done it."
Perhaps the reader may like to discover the number of that house.

Here is what I tried:
Let n = the number of the house that the Belgian lived in
m = the total number of houses in that street
and 50<m<500

now








now how do i sove for n and m?

matt grime

By using the restrictions placed on how many houses there are. It is just trial and error from there, plus some other observations, like m^2+m is the product of coprime integers m and m+1.

murshid_islam

but Ramanujan supposedly solved it using continued fractions. but how?

i dont see how that helps.

arildno

Ramanujan solved every problem he met with continued fractions. Perhaps that's why we don't understand any longer how he got his results.

Sorry, just kidding, I'm out of here..

matt grime

So, if I tell you that ab is a perfect square and that a and b are coprime, you can't deduce anything at all about a and b?

murshid_islam

sorry, maybe it is incredibly stupid of me, but i can't see what you are suggesting.

murshid_islam

can anyone please help me a little more?

matt grime

Didn't we have long thread on getting you to prove that if p is a prime and p divides a^2 then p divides a? Doesn't this spark something. Just think of a composite squared, like 6^2=36. Now, how can you write 36 as a product of comprime numbers? Notice anything about them?

murshid_islam

so the coprimes must be perfect squares too? is that it?

3trQN

What is the difference between a perfect square and a square? (if any)

Its ambiguous to me.

CRGreathouse

"Perfect square" = "square of an integer"

, where , is a square but not a perfect square.

matt grime

Yes.

Now, you have 2n^2 = m(m+1).

Either m is even and m/2 and m+1 are prefect squares, or m is odd and m and (m+1)/2 are prefect squares. There aren't many perfect squares in the region of 50 to 500 to check, are there? (Yes, 'perfect square' means 'square of an integer', it is perhaps a silly distinction).

3trQN

Are all real numbers then square?

To me, square number = perfect square number.
It seems pointless to give all real numbers the distinction square. I think square should mean all real numbers with integer roots and perfect squares should be conserved for the set of perfect numbers, squared.

That is:
Perfect Numbers = {6,28,496,....}
Perfect square: {36,784,.......}

imo.

neutrino

The square of "perfect numbers" (integers) ARE perfect squares.

matt grime

why are you introducing real numbers?

murshid_islam

but the square roots of all perfect squares are NOT perfect numbers. :)