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91 (ninety-one) is the natural number following 90 and preceding 92.

In mathematics

Ninety-one is the twenty-seventh distinct biprime and the second of the form (7.q). The aliquot sum of 91 is 21 within the aliquot sequence (91,21,11,1,0) 91 being the fourth composite number in the 11-aliquot tree.

Ninety-one is a triangular number and a hexagonal number, one of the few such numbers to also be a centered hexagonal number, and it is also a centered nonagonal number and a centered cube number. It is a square pyramidal number, being the sum of the squares of the first six integers.

It is the smallest positive integer expressible as a sum of two cubes in two different ways if negative roots are allowed: 91 = 63+(-5)3 = 43+33. (See 1729 for more details).

It is also the smallest pseudoprime satisfying the congruence 3n = 3 mod n.

91 is a repdigit in base 9 (111).

In science

In astronomy,

Messier object M91, a magnitude 11.5 spiral galaxy in the constellation Coma Berenices

The New General Catalogue object NGC 91, a single star in the constellation Andromeda

The Saros number of the solar eclipse series which began on August 6, 159 BC and ended on 1175 October. The duration of Saros series 91 was 1334.2 years, and it contained 75 solar eclipses.

The Saros number of the lunar eclipse series which began on 161 May and ended on 1459 July. The duration of Saros series 91 was 1298.1 years, and it contained 73 lunar eclipses.

In other fields

Ninety-one is also:

In sports








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