information:extraction
Sequential summation

A pair of vertical bars | ⋅ | around a set of all-upper indices or all-lower indices (but not both), associated with contraction with another set of indices when the expression is completely antisymmetric in each of the two sets of indices:

means a restricted sum over index values, where each index is constrained to being strictly less than the next. More than one group can be summed in this way, for example:

When using multi-index notation, an underarrow is placed underneath the block of indices:

where