A method for calculating numerical values in a manner which can be interpreted as encoding places in a hierarchy, and are in a format convenient for storage and retrieval on computer systems. The numerical values are calculated by associating paths in a hierarchy with sub-sequences of terms of a mathematical series where an ordering of the sub-sequences according to the occurrence of the first terms of the sub-sequences in the mathematical series is the same as an ordering of the magnitude of the sums of the terms of the sub-sequences. Said numerical values can be conveniently stored as integer or floating-point data types commonly used in computer systems and as such assigned to appropriate data elements in a data structure which defines serial relationships between the items it stores. Thus this invention enables sequential data structures such as arrays, linked lists and databases to store and retrieve tree structure data efficiently.