171 (one hundred [and] seventy-one) is the natural number following 170 and preceding 172.

In mathematics

171 is the 18th triangular number and a Jacobsthal number. There are 171 transitive relations on three labeled elements, and 171 combinatorially distinct ways of subdividing a cuboid by flat cuts into a mesh of tetrahedra, without adding extra vertices. The diagonals of a regular decagon meet at 171 points, including both crossings and the vertices of the decagon. There are 171 faces and edges in the 57-cell, an abstract 4-polytope with hemi-dodecahedral cells that is its own dual polytope. Within moonshine theory of sporadic groups, the friendly giant \mathbb {M} is defined as having cyclic groups ⟨ m ⟩ that are linked with the function, This generates 171 moonshine groups within \mathbb {M} associated with f_{m} that are principal moduli for different genus zero congruence groups commensurable with the projective linear group.