Rajnish Kaushik 18 Posted January 5, 2014 1-Even number 2-Prime number 3-Not prime 4-Non of these Please explain yor answer 0 Share this post Link to post Share on other sites

imatfaal 2480 Posted January 5, 2014 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 3 Share this post Link to post Share on other sites

Strange 4196 Posted January 5, 2014 It is obviously not even. And probably not prime. In fact, not prime: http://www.wolframalpha.com/input/?i=is+1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111+prime 0 Share this post Link to post Share on other sites

imatfaal 2480 Posted January 5, 2014 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 0 Share this post Link to post Share on other sites

Strange 4196 Posted January 5, 2014 Please explain yor answer Please explain your question! 1 Share this post Link to post Share on other sites

Rajnish Kaushik 18 Posted January 6, 2014 thanks friends 0 Share this post Link to post Share on other sites

deesuwalka 1 Posted October 6, 2016 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 0 Share this post Link to post Share on other sites

renerpho 9 Posted October 8, 2016 (edited) 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 1 Share this post Link to post Share on other sites

imatfaal 2480 Posted October 10, 2016 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. 0 Share this post Link to post Share on other sites