51 (fifty-one) is a positive integer following 50 and preceding 52. Its ordinal form is written "fifty-first" or 51st.


In googology

  • In Greek-based number-naming systems, 51 is associated with prefix "henpentaconta-", and with prefix "unquinquaginti-" in Latin systems.
  • 51 is the 7th output of the Xi function, \(\Xi(7) = 51\)
  • 51 is equal to \(\text{Worm}(2)\) in Beklemishev's worms.


  1. OEIS, Sequence A001358. Accessed 2021-05-19.
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