You can run this with or without a numeric argument. I wouldn't recommend numeric arguments past 15; it becomes pretty processor bound.

$p=Math::BigFloat;$n=$ARGV[0]||10;$p->accuracy(400);$w=do{$w=$p->new;
$w+=1/$p->new($_)->bfac for 0..125;$w->bstr};for(0..length($w)-$n){$z
=substr$w,$_,$n;use Math::BigFloat;if(ip($z)){print"$z \@ $_$/";last}
}sub ip{($y)=@_;($s,$d)=(sqrt$y,2);{return if$y%$d==0;last if$d++>$s;
redo}1}

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<Reveal this spoiler or all in this thread>

Dave

In reply to A natural occurrence by davido

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