The following problem appeared as ENIGMA 1168 in New Scientist 5 January 2002
THRICE UNFACTORISED
Richard England
I have found a four-digit number such that it is impossible to
factorise the numbers formed by its first digit or last digit or first
two digits or middle two digits or last two digits or first three
digits or last three digits or all four digits. In other words all
those eight numbers are prime except that either or both of the
single-digit numbers may be unity.
Harry and Tom have also found such a four-digit number.
The four-digit numbers that we have found are all different; but
Harry's number uses the same digits as Tom's number, though
in a different order. Which four-digit number have I found?
We might use SQL to generate prime numbers - but it's easier and quicker
to get some from elsewhere such as
James Edwards at Bristol or
Chris Caldwell at the University of Tennessee.
The table prime contains the 1300 small primes including
all the four digit primes.
1.
Check that the tables primes exists by viewing all primes less than 50.
Results
2.
The SUBSTR function in Oracle may be used to extract digits.