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 a₀ = a₁ = 1, and thenceforth a_n = 1/n − 1(a₀² + a₁² + ... + a_{n − 1}²). This sequence continues 1, 1, 2, 3, 5, 10, 28, 154... (sequence in the OEIS). a₄₃ 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 a (original) McNugget number.

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

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

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