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Ivancevic_Applied-Diff-Geom

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Technical Preliminaries: Tensors, Actions and Functors 119tween 2−arrows, and so on up to n [Baez (1997); Baez and Dolan (1998);Leinster (2002); Leinster (2003); Leinster (2004)].More precisely, an n−category (for n ≥ 0) consists of:• 0−cells, or objects, A, B, . . .• 1−cells, or arrows, Af ✲ B, with a compositionAf ✲ Bg ✲ C = Ag◦f ✲ C• 2−cells, ‘arrows between arrows’, A∨ α❘ B, with vertical compositions(denoted by ◦) and horizontal compositions (denoted by ∗),✒grespectively given byfAAfαg ∨ ✲◆B = Aβ ✍∨hf∨ α❘ ✒gA ′ f ′f∨ β◦α❘ B✒hα ′❘ A ′′ = A∨ ✒g ′andf ′ ◦fα ′ ∗ α ❘ ∨ ✒g ′ ◦gA ′′• 3−cells, ‘arrows between arrowsfΓ ❘between arrows’, A α > β B (where the Γ−arrow goes in✒ ga direction perpendicular to f and α), with various kinds of vertical,horizontal and mixed compositions,• etc., up to n−cells.Calculus of n−categories has been developed as follows. First, thereis K 2 , the 2–category of all ordinary (or small) categories. K 2 has cate-

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