18
$\begingroup$

I was playing around with the squares and saw an interesting pattern in their differences.

$0^2 = 0$

      + 1

$1^2 = 1$

      + 3

$2^2 = 4$

      + 5

$3^2 = 9$

      + 7

$4^2 = 16$

      + 9

$5^2 = 25$

      + 11

$6^2 = 36$

      etc.

(Also, in a very related question, which major Math Research Journal should I contact to publish my groundbreaking find in?)

$\endgroup$
4
  • 13
    $\begingroup$ Could you send it to a social science journal instead? :-) $\endgroup$
    – Simon Nickerson
    Jul 20, 2010 at 21:42
  • 1
    $\begingroup$ You can also reverse your observation (which as Dan explains is very easy to show) to get the slightly harder result that 1+3+5+...+(2n-1) = n^2. $\endgroup$
    – Noah Snyder
    Jul 20, 2010 at 21:42
  • 1
    $\begingroup$ The comment about publishing in a major math research journal is presumably a joke, but it seems like some of the answers think it was meant seriously.... $\endgroup$
    – Nate Eldredge
    Feb 4, 2013 at 22:14
  • 1
    $\begingroup$ Maybe it should be published in a social media journal, instead! ;þ $\endgroup$
    – BMeph
    Mar 18, 2014 at 13:59

8 Answers 8

Reset to default
30
$\begingroup$

alt text

Dan's algebraic justification is correct, but you may get more intuition about why this is happening from the above picture. Each time you want to enlarge the square by one unit, you have to add an extra row, an extra column, and one more square to fill in the corner. These correspond directly to the $n+n+1=2n+1$ that Dan mentioned. And of course, $2n+1$ is how odd numbers look.

Looking at it from the other direction, you can use the same idea to convince yourself of Noah's claim that $1+3+5+\dots+(2n-1) = n^2$. Imagine the left-hand side as representing the upper sequence of green and orange squares. You add on first 1 tile, then 3, then 5, and so on to construct larger and larger squares. At each step, you are adding one row (n) and one column (another n), then removing that one tile in the top right corner where they overlap (for a total of 2n-1).

$\endgroup$
1
  • $\begingroup$ Very nice explanation, this kind of intuition is what I'm always looking for. $\endgroup$
    – jeremy radcliff
    May 7, 2015 at 17:05
29
$\begingroup$

A nice thing to notice.

It's basically because

$(n + 1) ^ 2 - n ^ 2 = 2n + 1$

$\endgroup$
1
  • 1
    $\begingroup$ +1: And this would be the induction step in a formal proof, after noting the initial case being that the sum of the first 1 odd numbers is 1 ^2. $\endgroup$
    – donroby
    Jul 21, 2010 at 1:17
3
$\begingroup$

to extend this idea, there is something i came up with in high-school (now i suck at even elementary math)... I'm sure someone had come up with this much earlier ..

"the Nth difference between Nth consecutive powers is equal to N!"

for N=2
1  4  9  16  <- Nth powers
  3  5  7     <- 1st Difference
   2  2       <- Nth Difference

for N=3

1  8  27  64  125
 7  19  37  61
  12  18  24
     6   6

for N=4

1  16  81  256  625  1296  2401
 15  65  175  369  671
   50  110  194  302
     60   84   108
       24    24

this can also be used to calculate the next number in the series eg: 7^4 = 24+108+302+671+1296 = 2401

i wish i could remember the proof too ... Yikes

so for N=2 difference is 2 hence leading to a sequence of odd numbers

$\endgroup$
2
  • $\begingroup$ This can be understood with the difference operator $\Delta f(x) = f(x+1)-f(x)$. Notice each time you apply it the degree drops so you would only be interested in keeping track of the highest degree term, $\Delta(x\mapsto x^n)(x) = nx^{n-1}+\mathcal O(x^{n-2})$. Apply this $n$ times and you'll see what happens. $\endgroup$
    – Myself
    Mar 18, 2011 at 9:06
  • $\begingroup$ See math.stackexchange.com/questions/201330/… for the proof $\endgroup$
    – Alex
    Oct 20, 2012 at 7:34
2
$\begingroup$

The easiest way to I think about the difference between consecutive powers ( squared cubed or whatever) is using binomial theorem.

For example :

$(n+1)^4 - n^4 = n^4 + 4n^3 + 6n^2 + 4n + 1 - n^4 = 4n^3 + 6n^2 + 4n + 1$, with the binomial coefficients found by Whatever method you prefer.

This correctly predicts e.g $2^4-1^4=15$.
And when used on squares gives $2n+1$.

$\endgroup$
2
$\begingroup$

Don't make this a paper. This has been done over the centuries by so many brilliant mathematicians.

Reading Gauss, Newton, Hardy, Littlewood, Ramanujam and so many other mathematicians over the years will tell you that everything that you imagine has already been done.

You have to read many papers for many years, understand them in depth to come up with new observations using those publications as your base.

But hey... don't stop finding stuff by yourself. It is a good thing to do this kind of stuff.

regarding difference between consecutive cubes, I am more interested in explaining to myself why $d(x^3)/dx = 3x^2$ whereas for natural numbers $(n+1)^3 -n^3 = 3n^2 + 3n + 1$ all I can think of is

$lim (3n^2 + 3n + 1)$ =~ $3n^2$ as $n^2$ dwarfs $3n$. $n\rightarrow \inf$

$\endgroup$
2
  • 1
    $\begingroup$ This is one of the first questions on this site. I doubt the OP actually wanted to publish this. $\endgroup$
    – Asaf Karagila ♦
    Oct 19, 2012 at 12:46
  • 12
    $\begingroup$ 'Everything you imagine has already been done'. While I understand where the sentiment is coming from, I cannot imagine a worse statement to greet someone coming into mathematics with. $\endgroup$
    – Steven Stadnicki
    Feb 4, 2013 at 22:14
0
$\begingroup$

In general, $a^2-b^2=(a-b)(a+b)$ So $(x+1)^2-x^2=(x+1-x)(2x+1)=2x+1$ So all odd numbers are generated.

$\endgroup$
-1
$\begingroup$

o.k. you see the difference between the squares, but you dont understand why. I also have noticed this differnce, but i took it a step further when i came up with equation (a+1)^2- (a)^2 + 2 + (a+1)^2 = (a+2)^2. i asked my question which you can see in the section tagged "consecutive squares". which was, "if it was part of a bigger equation but found out it wasn't". none the less it took the differences of a certain square and corelated it to the next consecutive square (such as the fact that 1^2=1, and 2^2=4 and the difference between them, as shown in the chart above, is 3 sure, great! but what of it ?), well if you add 2 to that difference than add it back to the (2^2, or 4) the larger consecutive square. Then you get 9 which is the next cosecutive square, (3^2). That is the concept of the equation above, technically i didn`t explain that specific aspect of the difference of consecutive squares, but i did explain the general correlation between consecutive squares.

$\endgroup$
1
  • $\begingroup$ o.h. by the way this was definitiley not discovered by you and as i said earlier this is only the begining to my real, complete equation, i didn't get any realization for my equation so if you think your going to get any thing for this concept than your mistaken. i came up with this simple concept years ago and then took it farther and made it a actual equation this is nothing more than a base to the real equation that i came up with. I credit none of my discovery to you, if they didn't even except what i came up with. there just going to laugh at you. $\endgroup$
    – user7911
    Mar 7, 2011 at 16:03
-2
$\begingroup$

The difference between two consecutive cubes x^3 and (x+1)^3 will be 3x^2+3x+1. It gets confusing after that... for consecutive hypercubes, it's 4x^3-6x^2+4x-1.

$\endgroup$
1
  • 2
    $\begingroup$ This may be an interesting observation, but it doesn't really answer the question... $\endgroup$
    – Ben Millwood
    Sep 21, 2012 at 17:05

Not the answer you're looking for? Browse other questions tagged or ask your own question.