Rajnish Kaushik Posted January 5, 2014 Share Posted January 5, 2014 1-Even number 2-Prime number 3-Not prime 4-Non of these Please explain yor answer Link to comment Share on other sites More sharing options...
imatfaal Posted January 5, 2014 Share 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 Link to comment Share on other sites More sharing options...
Strange Posted January 5, 2014 Share 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 Link to comment Share on other sites More sharing options...
imatfaal Posted January 5, 2014 Share 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 Link to comment Share on other sites More sharing options...
Strange Posted January 5, 2014 Share Posted January 5, 2014 Please explain yor answer Please explain your question! 1 Link to comment Share on other sites More sharing options...
Rajnish Kaushik Posted January 6, 2014 Author Share Posted January 6, 2014 thanks friends Link to comment Share on other sites More sharing options...
deesuwalka Posted October 6, 2016 Share 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 Link to comment Share on other sites More sharing options...
renerpho Posted October 8, 2016 Share 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 Link to comment Share on other sites More sharing options...
imatfaal Posted October 10, 2016 Share 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. Link to comment Share on other sites More sharing options...
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