# Number Sequences

1. Oct 26, 2008

### exodian

Hello all,

I was wondering if you all could help me with a small problem.

Me and a friend had a discussion about a number sequence i found on http://www.fibonicci.com/en/number-sequences

1, 3, 7, 11, 13 ...

He says the next correct number is 27 and I say it's 17.

This forum seems full of very intelligent people, so i thought i post it here to get a definite answer! So which is the only correct one? :)

2. Oct 26, 2008

### dirk_mec1

it is +2 +4 +4 so the next number is 17.

3. Oct 26, 2008

### exodian

Ye, i kno! :D but he goes mumbeling something about primenumbers 10+n^2 dont really understand it and says the next number is 27

Is this correct or not?

So if all of you people say its 17 then i win hehe ^^

4. Oct 26, 2008

The underlying assumption is that the current pattern continues indefinitely - you are assuming that the "+2 +4 + 4 +2 +4 +4" will go on forever. if you can make that assumption, then the answer would be 17. If you can't make that assumption, there is no single "correct" answer.

5. Oct 26, 2008

### Hurkyl

Staff Emeritus
The next number is 0. They obviously meant the sequence
1, 3, 7, 11, 13, 0, 0, 0, 0, 0, 0, ....

6. Oct 28, 2008

### dirk_mec1

What kind of sequence is this?

7. Oct 28, 2008

### HallsofIvy

Staff Emeritus
An arbitrary one.

8. Oct 28, 2008

### Tac-Tics

A slightly less arbitrary sequence beginning this way is:
1, 3, 7, 11, 13, 17, 31, 33, 37, 71, 73, 77, 111, 113, 117, ...

Which is the sequence of numbers in base three with digits 1, 3, and 7 used instead of 0, 1, and 2.

"Guess which sequence I'm thinking of"-type questions are lame because no finite number of initial members will uniquely specify it.

9. Oct 28, 2008

### CRGreathouse

Your friend refers to http://www.research.att.com/~njas/sequences/A114273 [Broken].

There is no single 'correct' answer. There are uncountably many integer sequences that start 1, 3, 7, 11, 13.

Last edited by a moderator: May 3, 2017
10. Oct 30, 2008

### Riogho

Yes, but he is talking about the fibbonacci numbers, in which case the next answer would be 27.

11. Oct 30, 2008

### Tac-Tics

But nothing about the numbers he gave shows that it *is* the fibonacci sequence.