diff --git a/doc/src/compute_sna_atom.rst b/doc/src/compute_sna_atom.rst
index b17db625f8..d604ca9d29 100644
--- a/doc/src/compute_sna_atom.rst
+++ b/doc/src/compute_sna_atom.rst
@@ -451,7 +451,10 @@ piece of python code:
            for j in range(j1-j2,min(twojmax,j1+j2)+1,2):
                if (j>=j1): print j1/2.,j2/2.,j/2.
 
-For even twojmax = 2(*m*\ -1), :math:`K = m(m+1)(2m+1)/6`, the *m*\ -th pyramidal number. For odd twojmax = 2 *m*\ -1, :math:`K = m(m+1)(m+2)/3`, twice the *m*\ -th tetrahedral number.
+There are :math:`m(m+1)/2` descriptors with last index *j*,
+where *m* = :math:`\lfloor j \rfloor + 1`.
+Hence, for even *twojmax* = 2(*m*\ -1), :math:`K = m(m+1)(2m+1)/6`, the *m*\ -th pyramidal number,
+and for odd *twojmax* = 2 *m*\ -1, :math:`K = m(m+1)(m+2)/3`, twice the *m*\ -th tetrahedral number.
 
 .. note::