This revised post follows up on reciprocal (or harmonic or parallel) addition mentioned in a previous post *here*.

*Reciprocal arithmetic* is based on an automorphism that interchanges the zero with the infinite and the greater-than-one with the less-than-one: 0 ↔ ∞ and *x* ↔ 1/*x*. So zero becomes the new inaccessible number and infinity becomes the new additive unit. That is,

*x* ⊞ *y* := *g*^{−1}(*g*(*x*) + *g*(*y*))

*x* ⊡ *y* := *g*^{−1}(*g*(*x*) ∙ *g*(*y*))

etc., where *g*(*x*) = 1/*x* with x ≠ 0.

The identity element is ∞ if it is included, so that x ⊡ ∞ = x and x ⊡ ∞ = ∞. The additive inverse of x is −x, and the multiplicative inverse is x^{−1}.

Reciprocal arithmetic is isomorphic to the ordinary numerator operations exchanged with their denominator counterparts. It is like counting down from a maximum, in which an increment of one reduces the amount slightly.

Reciprocal addition (also known as parallel addition) is defined as a power operation:

with the understanding that (1/0) → ∞ and (1/∞) → 0. An intermediate value may be zero but not a final value.

Simple reciprocal addition is thus defined as:

The reciprocal additive unit is infinity instead of zero: x ⊞ ∞ = ∞. Reciprocal increment is:

Reciprocal addition is commutative, associative, and distributes with multiplication. See Kent E. Erickson, “A New Operation for Analyzing Series-Paralled Networks,” *IRE Trans. on Circuit Theory*, March 1959, pp.124-126, See also *here* and *here*.

Reciprocal subtraction is defined as:

Reciprocal multiplication is defined as:

Reciprocal division is defined then as:

Reciprocal exponentiation is defined as: