use strict;
use warnings;
use Math::Trig;  # why settle for less (precision)?

my ( @sigmas, @rads );
my ( $a1, $c ) = ( 1.18, .28e6 );

my $i = 1;
while ( $i < 20 ) {
    my $r = $i * 1e-6;
    push @rads, $r;
    push @sigmas, pi * ($r**2) * $a1 * (1-exp(-$c*$r));
    $i += 2;
}
while ( $i < 100 ) {
    my $r = $i * 1e-6;
    push @rads, $r;
    push @sigmas, 2 * pi * ($r**2);
    $i += 2;
}

for $i ( 0 .. $#rads ) {
    printf "rad= %8.3g  sigma= %8.3g\n", $rads[$i], $sigmas[$i];
}