Sequence with 2 parameters (?)

[Cinitiator]

Homework Statement

Could anyone please help me understand this notation? What is "it" under the variables? Is it some kind of sequence notation with 2 dimensions, or sequence notation which has the sequence member number multiplied by some other variable? What is this?

Also, what could 0 over z mean, other than 0 exponent?

Homework Equations

The Attempt at a Solution

Tried Google search without help.

 

[micromass]

This is truly impossible to answer without knowing the exact source of this equation.

 

[Simon Bridge]

I'll second that: what's the context?
Subscripts and superscripts are used for things besides iterations and powers... U²³⁸ is not normally read as the 238th power of U for example, and the x component of the initial velocity can be noted as v_0x... giving two subscripts.