# An amazing calculation!

1. Mar 1, 2009

### Archosaur

Guess what! Through complete and precise calculations, I have discovered that 73+47=5! Isn't that crazy? I stand firm by my conclusion and beseech anyone to challenge me!

I'm actually serious, by the way. I know how some of you might take this, but I'm not lying.

I would bet my life on the truthfulness of that equality.

2. Mar 1, 2009

### John Creighto

I'm not going to check your calculation right now but if it is true then why is it significant?

3. Mar 1, 2009

### cristo

Staff Emeritus
Well of course it's true... but, what is the point of your thread?

4. Mar 1, 2009

### arildno

Even better, did you know that 2=2!!!!!!!!!!!!!!!!!!
I think that is pretty amazing..

5. Mar 1, 2009

### Mentallic

For a few seconds there I had to think about this also... The equality isn't simple enough at first glance to be another one of "those" threads.

6. Mar 1, 2009

### HallsofIvy

What is not simple? Pick any integer, take its factorial and you can always find two numbers that add up to it.

Archosaur, it is also true that 55+ 65= 5! or the 23+ 97= 5!. What is special about 73 and 47?

7. Mar 1, 2009

### arildno

You really are uninsightful today, Halls!

First off, 73 and 47 are PRIMES, but even more amazing, their difference is the only integer that is squeezed in between a cube and a square, as Ramanujan proved!!

8. Mar 1, 2009

### Mentallic

arildno (post#4) has given us a special equality, which happens to be very simple. (I wouldn't stop twice to think about its elegance).

Arildno, if this equality has some special features as you have stated, please explain in a little more detail what 'an integer squeezed between a square and a cube' means

9. Mar 1, 2009

### muppet

25= 5 squared; 27=3 cubed. so the difference between them is one less than a cube and one more than a square.
Is it really that amazing that they're both primes? Goldbach's conjecture has been numerically verified for every number up to the order of 10^18 (according to wikipedia!). 5! is an even number...

10. Mar 1, 2009

I have just checked "the penguin dictionary of curious and interesting numbers author David Wells" and 120 appears there.It is the 15th triangular number the 8th tetrahedral number,the list goes on.

11. Mar 1, 2009

### Archosaur

Okay okay okay, so you knew the trick right away...

I knew I should have posted this in the physics section....

See? You were supposed to think that I was just exclaiming that 73+47=5, but you all immediately identified the factorial. I'll go find some history majors to try it on.

And yes, 73 and 47 are special, kinda. Firstly, they were the first prime numbers that came to my head that add to 120, but then I noticed that they are the 20th and 15th primes, respectively and 20-15 is 5, which is the number that I took the factorial of.

Anyway, sorry for the attempted deception :)

12. Mar 1, 2009

### cristo

Staff Emeritus
No.. you should have posted this in the general discussion section, or better still not posted it at all. :uhh:

13. Mar 1, 2009

### arildno

You should try it on forums devoted to social "sciences", or for nurses, or for primary school teachers and suchlike. I'm sure there will be much flutter and chatter over your problem in such places.

14. Mar 1, 2009

### confinement

If the postfix factorial notation is not interpreted as a punctuation mark then your sentence is a grammatical failure. It would be nice if you could sneak some punctuation into the original joke to overcome this flaw.

15. Mar 1, 2009

### alxm

Why? To give a brilliant demonstration of smart-assedness?

The fact that you know "!" can be interpreted as a mathematical operator doesn't make you smarter than those who don't. Which is pretty obvious given that very few would be impressed by remarking that "73+47=120" or "73+47=2*3*4*5"

16. Mar 1, 2009

### confinement

(1) The exclamation point for factorials has been used for over 200 years, and in the US it is generally taught in middle school.

(2) There are dozens of riddles of this type that have to do with obscure meanings of words, whereas "!" for factorial is the secondary usage that most pople will think of if pressed.

17. Mar 1, 2009

### Archosaur

Haha, yes I did notice that. I thought about sneaking a period in after the "!", but that would make it even more obvious... Oh, I could write "...73+47=5!!!" and say that it's "73+47=5*4*3*2" and "!!" which is still grammatically incorrect, but maybe it's closer.

Whoa dude. I'm just playing around. Of course that doesn't make me smarter and I never said it did. No need to go on the defensive. Lighten up a little.

18. Mar 1, 2009

### Archosaur

at least you were nice about it...

19. Mar 1, 2009

### qntty

20. Mar 1, 2009

### cfl25

you guys need to do a better job at spotting trolls when you see them