2019-11-23, 19:58   #327
sweety439

Nov 2016

17·149 Posts

Quote:
 Originally Posted by sweety439 Not including the single-digit primes, proof of that these sets are complete: b=2: we obtain the 2-digit primes 10 and 11, since for any prime p > 11, p must start and end with digit 1, and we have 11 <<< p, thus the 2-kernel {10, 11} is complete. b=3: we obtain the 2-digit primes 10, 12 and 21, for any prime p > 21, if p end with 2 and 12 !<<< p, then p must contain only 0 and 2, thus p is divisible by 2 and > 2, thus not prime, therefore, p must end with 1 (p cannot end with 0, or p is divisible by 10 and not prime), since 21 !<<< p, p must contain only 0 and 1, but since 10 !<<< p and p cannot have leading zeros, thus p can only have the digit 1, i.e. p is a repunit, and the smallest repunit prime is 111, thus completed the 3-kernel {10, 12, 21, 111}. b=4: we obtain the 2-digit primes 11, 13, 23 and 31, for any prime p > 31, if p end with 3 and 13 !<<< p and 23 !<<< p, then p must contain only 0 and 3, thus p is divisible by 3 and > 3, thus not prime, therefore, p must end with 1 (p cannot end with 0 or 2, or p is divisible by 2 and not prime), since 11 !<<< p and 31 !<<< p, thus p (before the final digit 1) must contain only 0 and 2, and we obtain the prime 221, since p cannot have leading zeros, the remain case is only 2{0}1, but all numbers of the form 2{0}1 are divisible by 3 and > 3, thus not prime, thus we completed the 4-kernel {11, 13, 23, 31, 221}. b=5: we obtain the 2-digit primes 10, 12, 21, 23, 32, 34 and 43, for any prime p>43: p end with 1 --> before this 1, p cannot contain 2 --> if p end with 11, then we find the prime 111, and all other primes contain at most two 1, ... p end with 2 --> before this 2, p cannot contain 1 or 3 --> p only contain 0, 2 and 4 --> p is divisible by 2 and > 2 --> p is not prime (thus, 12 and 32 are the only such primes end with 2) p end with 3 --> before this 3, p cannot contain 2 or 4 --> p only contain 0, 1 and 3 --> we obtain the primes 133 and 313, thus other primes p cannot contain both 1 and 3 (before the final digit 3), and since p cannot have leading zeros, if p begin with 1, then p is of the form 1{0,1}3 --> it must be of the form {1}3 (to avoid the prime 10) --> but 113 is not prime and all primes except 111 contain at most two 1 --> this way cannot find any primes, if p begin with 3, then p is of the form 3{0,3}3 --> p is divisible by 3 and > 3 --> p is not prime (thus, all such primes end with 3 are 23, 43, 133 and 313) p end with 4 --> before this 4, p cannot contain 3 --> since all primes > 2 are odd, p must contain at least one 1 --> we obtain the prime 414 and we know that no 2 can before this 1 (to avoid the prime 21) and no 0 or 2 can after this 1 (to avoid the primes 10 and 12) --> 1 must be the leading digit (since p cannot have leading zeros, and no 2, 3, 4 can before this 1 (to avoid the primes 21, 34 and 414, respectively) --> p must be of the form 1{4} or 11{4} (since all primes except 111 contain at most two 1) --> and we obtain the prime 14444 (thus, all such primes end with 4 are 34, 414 and 14444)
b=6: we obtain the 2-digit primes 11, 15, 21, 25, 31, 35, 45 and 51, for any prime p > 51:

* if p end with 5 --> before this 5, p cannot contain 1, 2, 3, or 4, however, all numbers of the form {0, 5}5 is divisible by 5 and cannot be prime

* thus, p can only end with 1 --> before this 1, p cannot contain 1, 2, 3, or 5 --> before this 1, p can only contain the digits 0 and 4 --> since the first digit cannot be 0, it can only be 4 --> p is of the form 4{xxx}1 --> x should contain at least one 4 (or p is of the form 4{0}1, and 4{0}1 is divisible by 5) --> thus we can assume p is 4{xxx}4{yyy}1

* if y contain at least one 0, then we have the prime 4401

* if y contain at least one 4, then we have the prime 4441

* if y is empty:

** if x contain at least one 4, then we have the prime 4441

** if x does not contain 4 (thus contain only the digit 0), then we have the prime 40041

The 6-kernel is complete!!!