- Thread starter mitchfel
- Start date

- Joined
- Apr 12, 2005

- Messages
- 10,989

n * (n+1)I tried a lot of numbers that’s divisible by 11 and substitute it to the n. The statement is true therefore it is not counterexample. I need a whole number if you substitute it to n, the result would be not a prime number. Please help me

Consecutive Integers.

One of them is divisible by 11.

Maybe 10 and 11 or 11 and 12?

Maybe 21 and 22 or 22 and 23?

Maybe 32 and 33 or 33 and 34?

There are other types of counter examples, but this variety seems the most obvious.

- Joined
- Jun 18, 2007

- Messages
- 22,405

If you select n = 10 or n = 21 or n = 32 what do get? Can you generalize this process (n = ?)

- Joined
- Dec 30, 2014

- Messages
- 8,290

Wait wait, finally a problem that I can answer! How about 10 and 11 or even 11 and 12!!You don't need counterexample. It takes only one. What sort fo search have you conducted?S

n * (n+1) Just looks SO tempting. Can you think of two consecutive integers where one of them happens to be divisible by 11?

- Joined
- Jan 27, 2012

- Messages
- 6,704

A "brute strength method" is to simply start trying whole numbers:If n is a whole number, then n^2+ n + 11 is a prime number. Give me counterexamples of it

0 0+ 0+ 11= 11, prime

1 1+1+ 11= 13, prime

2 4+ 2+ 11= 17, prime

3 9+ 3+ 11= 23, prime

4 16+ 4+ 11= 31, prime

5 25+ 5+ 11= 41, prime

6 36+ 6+ 11= 53, prime

7 49+ 7+ 11= 67, prime

8 64+ 8+ 11= 83, prime

9 81+ 9+ 11= 101, prime

10 100+ 10+ 11= 121= 11*11! NOT PRIME!

If I were as smart as I like to think I am, I would have seen that "11" at the end and thought "11 is itself prime- the only way to factor 11 is "11*1" so immediately try 11^2+ 11+ 11= 11(11+ 1+ 1)= 11(13).

Another answer is

11 121+ 11+ 11= 143= 11*13 which I should have been able to get right away!

It is interesting that, after all those prime numbers, we get two non-primes in succession. What about 12? 144+ 12+ 11= 167 which is prime.

- Joined
- Jun 18, 2007

- Messages
- 22,405

A "brute strength method" is to simply start trying whole numbers:

0 0+ 0+ 11= 11, prime

1 1+1+ 11= 13, prime

2 4+ 2+ 11= 17, prime

3 9+ 3+ 11= 23, prime

4 16+ 4+ 11= 31, prime

5 25+ 5+ 11= 41, prime

6 36+ 6+ 11= 53, prime

7 49+ 7+ 11= 67, prime

8 64+ 8+ 11= 83, prime

9 81+ 9+ 11= 101, prime

10 100+ 10+ 11= 121= 11*11! NOT PRIME!

If I were as smart as I like to think I am, I would have seen that "11" at the end and thought "11 is itself prime- the only way to factor 11 is "11*1" so immediately try 11^2+ 11+ 11= 11(11+ 1+ 1)= 11(13).

Another answer is

11 121+ 11+ 11= 143= 11*13 which I should have been able to get right away!

It is interesting that, after all those prime numbers,we get two non-primes in succession.What about 12? 144+ 12+ 11= 167 which is prime.

Similar equations (for generation of prime numbers) are:

n

n

Last edited: