lets see

$A = ($a*($p1-$p2) + $b*($pT-$p1)/(-2));
$B = ($p1*($a*($p2-$p1) + $b*($pT-$p1)-$k*$d2-$d1i*$k));
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In your script $p1 = $p2, so this is the same as:

 $A = -0.5*$b*($pT-$p1);
 $B = $b*($pT-$p1)-$k*($d2-$d1i);
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What's the purpose of the var $p2?. Maybe you want to say:

$p2 = $p1; 
$p1 += .01;
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Instead

 $p1 += .01; $p2 = $p1; ?

Check that your equations are valid