### Blatantly stolen from Wikipedia

**Forty-three** is the 14th smallest
prime number.
The previous is forty-one,
with which it comprises a twin prime,
and the next is forty-seven.
43 is the smallest prime that is not a Chen prime.
It is also the third Wagstaff prime.

43 is the fourth term of Sylvester's sequence, one more than the product of the previous terms (2 × 3 × 7).

43 is a centered heptagonal number.

Let a(0) = a(1) = 1, and thenceforth a(n) = (a(0)^{2} + a(1)^{2} + ... + a(n-1)^{2}) / (n-1).
This sequence continues 1 1 2 3 5 10 28 154... (sequence
A003504 in OEIS). Amazingly, a(43) is the first term of this sequence that is not an integer.

43 is a Heegner number.

43 is a repdigit in base 6 (111).

43 is the largest natural number that is not an (original) McNugget number.

This is the smallest number expressible as the sum of 2, 3, 4, or 5 different primes:

- 43 = 41 + 2
- 43 = 11 + 13 + 19
- 43 = 2 + 11 + 13 + 17
- 43 = 3 + 5 + 7 + 11 + 17.

4 | 15 | 17 |
7 |

5 | 19 | 13 |
6 |

20 | 9 | 2 |
12 |

14 | 0 | 11 |
18 |

The date magic square at right illustrates the magic constant as the sum of four primes:

When taking the first six terms of the Taylor series
for computing e, one obtains

which is also five minus the fifth harmonic number.