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Math::NumSeq::PrimeIndexPrimes -- primes with prime number indexes

=head1 SYNOPSIS

 use Math::NumSeq::PrimeIndexPrimes;
 my $seq = Math::NumSeq::PrimeIndexPrimes->new;
 my ($i, $value) = $seq->next;

=head1 DESCRIPTION

This is the sequence of primes which are at prime indexes,

    3, 5, 11, 17, 31, 41, 59, 67, 83, 109, 127, 157, 179, 191, ...
    starting i=1

The primes begin

    index  prime
      1      2
      2      3     <--+ primes at prime indexes
      3      5     <--+
      4      7        |
      5     11     <--+
      6     13        |
      7     17     <--+
      8     19

The primes marked "<--" have an index which is prime too.

=head2 Level

Optional C<level> controls how many repetitions of the prime indexing is to
be applied.  The level is based on the order of primeness by Neil Fernandez
in the PrimeIndexOrder sequence.

=over

L<http://www.borve.org/primeness/FOP.html>

=back

The default is level=2, asking for primes with an order of primeness
E<gt>=2.  level=1 gives all primes, and level=0 gives all integers.

The next higher level=3 restricts to primes whose index is prime, and then
in addition demands that prime is at an index which is prime.

    level => 3
    5, 11, 31, 59, 127, 179, 277, 331, 431, 599, ...

Successive levels filter further and the remaining values soon become quite
large.  For example level=11 starts at 9737333 (and is quite slow to
generate).

=head2 Level Exact

Optional C<level_type=E<gt>'exact'> asks for values which have exactly
C<level> as their order of primeness.

With the default level 2 this means primes whose index is a prime, but then
the index of that index is not a prime, ie. the iterations of prime index
stops there,

    level_type => 'exact', level => 2
    3, 17, 41, 67, 83, 109, 157, 191, 211, 241, 283, 353, ...

Here 11 is not in the sequence because its order of primeness is 3, since 11
is at index 5, 5 is at index 3, 3 is at index 2.

level_type=exact,level=1 means those primes which are at composite indexes.
This is all the primes which are not prime index primes, ie. primes not in
the default prime-index-primes sequence.

    level_type => 'exact', level => 1
    2, 7, 13, 19, 23, 29, 37, 43, 47, 53, 61, 71, 73, 79, ...

level_type=exact,level=0 means integers which have order of primeness 0,
which is the composites, ie. the non-primes.

=head1 FUNCTIONS

See L<Math::NumSeq/FUNCTIONS> for behaviour common to all sequence classes.

=over 4

=item C<$seq = Math::NumSeq::PrimeIndexPrimes-E<gt>new (level =E<gt> $n)>

Create and return a new sequence object.

=item C<$i = $seq-E<gt>value_to_i_estimate($value)>

Return an estimate of the i corresponding to C<$value>.  C<$value> can be
any size.

=back

=head1 Formulas

=head2 Value to i Estimate

The i for a given value can be estimated by applying the prime-to-i estimate
of the plain primes sequence (L<Math::NumSeq::Primes/Value to i Estimate>)
according to the C<level> parameter.

    repeat level many times
      value = Primes value_to_i_estimate(value)

    if level_type eq "exact"
      value = value - Primes value_to_i_estimate(value)

    i_estimate = value

For example the default level=2 prime index primes applies the Primes
estimate twice.  A given value is presumed to be a prime, it's index is
estimated.  Then that index also has to be a prime (so the original value is
a prime index prime), and the index of that is again estimated by the Primes
module.

For C<level_type=E<gt>'exact'> the final index must be a composite, as
opposed to "minimum" where it can be either prime or composite.  That
restriction means an extra final Composite value to i, derived from the