43 (forty-three) is the natural number following 42 and preceding 44.
43 is a prime number, and a twin prime of 41. 43 is the smallest prime that is not a Chen prime. 43 is also a Wagstaff prime, and a Heegner number. 43 is the fourth term of Sylvester's sequence.
43 is the largest prime which divides the order of the Janko group J<sub>4</sub>.
Dutch mathematician Hendrik Lenstra wrote a mathematical research paper discussing the properties of the number, titled Ode to the number 43.
Lenstra, Hendrik (2009). Ode to the number 43 (In Dutch). Nieuw Arch. Wiskd. Amsterdam, NL: Koninklijk Wiskundig Genootschap (5) 10, No. 4: 240-244.