TASK #1 - Truncatable Prime

Write a script to generate first 20 left-truncatable prime numbers in base 10.

In number theory, a left-truncatable prime is a prime number which, in a
given base, contains no 0, and if the leading left digit is successively
removed, then all resulting numbers are primes.

Example

9137 is one such left-truncatable prime since 9137, 137, 37 and 7 are
all prime numbers.

MY NOTES: Very easy, especially (tada) if you happen to have a prime
generating module that you've already used several times in these
challenges..


TASK #2 - Pentagon Numbers

Write a script to find the first pair of Pentagon Numbers whose sum and difference are also a Pentagon Number.

    Pentagon numbers can be defined as P(n) = n(3n - 1)/2.

Example

	The first 10 Pentagon Numbers are:
	1, 5, 12, 22, 35, 51, 70, 92, 117 and 145.

	P(4) + P(7) = 22 + 70 = 92 = P(8)
	but
	P(4) - P(7) = |22 - 70| = 48 is not a Pentagon Number.

MY NOTES: Ok, reasonably straight forward, calc first N Pentagon numbers,
form a sethash of them to allow lookups, then iterate over all pairs of
Pentagon numbers rejecting all pairs where the diff or sum isn't a Pentagon
number. The diff of any two of the first N Pentagon numbers is obviously
smaller, so may be looked up directly in the sethash.  The only tricky bit
concerns the SUM of any two of them, because that sum may be greater than
the biggest Pentagon number we know as yet, hence not in the sethash because
we haven't calculated such Pentagon numbers yet - not because it's not a
Pentagon number.  I can nearly see the structure of an adaptive solution,
that generates Pentagon numbers incrementally, but it's a bit tricky.
So instead let's Keep It Simple - just generate the first N Pentagon numbers
and see if we can find any matching pair of those (where the sum is in the
first N Pentagon numbers).  If not, run the program again with a bigger value
of N.  Experimentation reveals that N=2400 finds the solution..