Table 1.

Glossary of notation.

symbol	meaning
A	cross-sectional area
vol	volume
M	mass
P	power (active power)
V	voltage
B	metabolic rate
D	dimension
t	time
b	scaling exponent
Nx	number of components of type x
L	length of an edge in a network
K	depth of a tree
k	subscript denoting a level in a hierarchical branching network (k=0 refers to the root)
β	branching ratio $N_{k+1}/N_k$ in a network (also called fanout)
γ	length ratio $L_k/L_{k+1}$
$w_k$	width of an edge at level k in the network (e.g. wire width)
ω	width ratio $w_k/w_{k+1}$
ρ	density (per unit area or volume)
λ	process width
net	subscript denoting an entire network
org	subscript denoting an entire organism
chip	subscript denoting an entire computer chip
c	subscript denoting the lowest level component in a branching structure (e.g. capillary)
tr	subscript denoting transistors