The number 11111..111111..11 (91)times is an?

[Rajnish Kaushik]

1-Even number

2-Prime number

3-Not prime

4-Non of these

Please explain yor answer

[imatfaal]

1-Even number

2-Prime number

3-Not prime

4-Non of these

Please explain yor answer

 

 

Not even and not prime. It is divisible by 1,111,111 amongst others

 

If R_n is the number 1....1 with n repetitions then it can be shown that if n is not prime (ie can be divided by a and b) then R_n is not prime and that R_n can be divided by R_a and R_b

[Strange]

It is obviously not even. And probably not prime.

 

In fact, not prime: http://www.wolframalpha.com/input/?i=is+1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111+prime

[imatfaal]

It is obviously not even. And probably not prime.

 

In fact, not prime: http://www.wolframalpha.com/input/?i=is+1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111+prime

 

I had forgotten about wolfram alpha - it can give me the actual result to my divisor 1,111,111

1000000100000010000001000000100000010000001000000100000010000001000000100000010000001 x 1111111

That is such a cool looking sum; and if you think about it - the pattern makes total sense

[Strange]

Please explain yor answer

 

Please explain your question!

[Rajnish Kaushik]

thanks friends

[deesuwalka]

1. It's not an even number because it ends with 1, it's an odd number

3. It's not a prime number because it's divisible by 1,111,111 other than 1

[renerpho]

Primes of that form are are quite rare, see https://oeis.org/A004023 to find numbers of the form [math]111 \dots 111=\frac{10^n-1}{9}[/math] that are prime.
It turns out that this number is prime for n=2, 19, 23, 317, 1031, 49081, 86453, 109297 or 270343.

Edited October 8, 2016 by renerpho

[imatfaal]

Primes of that form are are quite rare, see https://oeis.org/A004023 to find numbers of the form [math]111 \dots 111=\frac{10^n-1}{9}[/math] that are prime.

It turns out that this number is prime for n=2, 19, 23, 317, 1031, 49081, 86453, 109297 or 270343.

 

Gotta love Sloane's. I hadn't realised they were so rare - just that it was relatively simple to show if the number had to be compound; obviously it is always simpler to show that a number is compound than to show that it is prime but in this case the test to show compound nature was so cute.