\(A_\infty \)-Category Theory

3 Stasheff Data

Definition 6 Operation target degree
#

Given input degrees \((d_0,\dots ,d_{n-1})\), the output degree of the \(n\)-ary operation is

\[ \sum _i d_i + (2-n), \]

where the final summand is interpreted using the grading shift .

Definition 7 Stasheff target degree
#

The common degree of the arity-\(n\) Stasheff relation is

\[ \sum _i d_i + (3-n) \]

in the grading index.

Definition 8 Composable morphism type
#

For a string of objects and prescribed degrees, is the \(i\)-th graded hom-module in that composable string.

Definition 9 Operation target type

For a composable string of objects and degrees, this is the graded hom-module in which the corresponding multilinear operation takes values.

Definition 10 Indexed Stasheff term

Fix object-indexed operations \(m_n\). For valid indices \((r,s)\), is the term obtained by first applying the inner \(s\)-ary operation in positions \(r,\dots ,r+s-1\) and then applying the outer operation to the collapsed string.

Definition 11 Stasheff sign parity
#

For a valid pair \((r,s)\), this is the parity

\[ |a_{r+s}| + \cdots + |a_{n-1}| - (n-r-s) \]

computed in .

Definition 12 Stasheff sign
#

The integer sign attached to \((r,s)\) is \((-1)\) raised to the parity recorded by .

Definition 13 Indexed Stasheff sum

The arity-\(n\) Stasheff sum is the signed sum of all valid composites of an inner operation followed by an outer operation.

Definition 14 Indexed Stasheff identities
#

An object-indexed family of multilinear operations satisfies the Stasheff identities if every indexed Stasheff sum vanishes.