# Problem 14.3a

A First Course in String Theory

14.3 Massive level in the open superstring.

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$b_i^2 = 0$

$b_i b_j = - b_j b_i$

$i_1 \ne i_2$

Since the sign is ignored, the number of possible $b^i b^j$ states is $\frac{1}{2} (8) (7) = 28$.

The number of possible $b^i b^j b^k$ states is $\frac{1}{6} (8) (7) (6) = 56$.

The number of possible $b^i b^j b^k b^l$ states is ${8 \choose 4} = 70$.

— Me@2015-09-27 10:40:30 AM