Prime posting thread, part 2. (With a catch.)

[CRGreathouse]

Originally Posted by 3.14159

The only set of numbers one can really use is 1 and 0, for a prime that has no substrings that are prime.

I don't believe you've proven that.

[Batalov]

[pedantic]The 'covering set' for 4-and-9s is 449, 499, 9949, 94949. [/pedantic]

The first two primes do not exclude the third, but after the third the only suriving pattern is 949...49, but unfortunately it immediately dies out with 94949.

[CRGreathouse]

Originally Posted by 3.14159

That is; That is, 2 and 1, 3 and 1, 4 and 1, 5 and 1, 6 and 1, 7 and 1, 8 and 1, 9 and 1, 3 and 2, 5 and 2, 7 and 2, 9 and 2, 4 and 3, 5 and 3, 7 and 3, 5 and 4, 7 and 4, 9 and 4, 6 and 5, 7 and 5, 8 and 5, 9 and 5, 7 and 6, 8 and 7, 9 and 7, 9 and 8, will all have a prime substring, given that it's over 15-20 digits. (Or will you be willing to provide a counterexample?)

If you're considering 2, 3, 5, and 7, does that mean we're allowed to have embedded primes of length 1?

[3.14159]

Originally Posted by CRGreathouse

If you're considering 2, 3, 5, and 7, does that mean we're allowed to have embedded primes of length 1?

Yes. Of course it does.

[science_man_88]

Originally Posted by Batalov

[pedantic]The 'covering set' for 4-and-9s is 449, 499, 9949, 94949. [/pedantic]

The first two primes do not exclude the third, but after the third the only suriving pattern is 949...49, but unfortunately it immediately dies out with 94949.

did he say it had to be the first prime in a sequence ? if so then I misread. will 94 adding on the start ever be prime again ?

[3.14159]

Originally Posted by science_man_88

did he say it had to be the first prime in a sequence ? if so then I misread. will 94 adding on the start ever be prime again ?

94949494949494949 is prime.

[science_man_88]

Originally Posted by 3.14159

94949494949494949 is prime.

I know what he means now pi the first one is the highest one that can be done without some part of it being prime. I realize that now.

[science_man_88]

for(x=0,9;y=0,9;for(z=1,30,if(isprime(),print()))

is there anyway other than vectors that i can test strings of x and y representing numbers and print the biggest one ?

[science_man_88]

Originally Posted by 3.14159

The digits have to be coprime. 6 and 9 will always be divisible by 3.

4 and 9; Not possible. (499.. substrings will be prime; 499, 4999, 49999.)
8 and 9; Not possible. (899.. substrings will be prime; 89, 8999, 89999999.)

The only set of numbers one can really use is 1 and 0, for a prime that has no substrings that are prime.

what about testing more first...

if you look at it the possible combos for those 2 are

494
499
449
444
994
999
949
944
94
99
49
44

so only 2 left that are prime of these: continue for others

[Batalov]

Originally Posted by 3.14159

94949494949494949 is prime.

... which has a prime substring 94949.

[3.14159]

Using 5 and 7;

75757575757575757575757575757575757575757575757575757575757575757575757575757 is prime. That might not have any prime substrings, beside 757.