43 (number)

43 (forty-three) is the natural number following 42 and preceding 44.

Mathematics
Forty-three is the 14th smallest prime number. The previous is forty-one, with which it comprises a twin prime, and the next is 47. 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$n$ = $1⁄n − 1$(a$0$$2$ + a$1$$2$ + ... + a$n − 1$$2$). This sequence continues 1, 1, 2, 3, 5, 10, 28, 154... . a$43$ is the first term of this sequence that is not an integer.

43 is a Heegner number.

43 is the largest prime which divides the order of the Janko group J4.

43 is a repdigit in base 6 (111).

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

43 is the smallest prime 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.

43 is the smallest number with the property 43 = 4*prime(4) + 3*prime(3). Where prime(n) is the n-th prime number. There are only two numbers with that property, the other one is 127.

When taking the first six terms of the Taylor series for computing e, one obtains
 * $$\sum_{i=0}^{5}\frac{1}{i!}=\frac{163}{60}=2+\frac{43}{60},\ $$

which is also five minus the fifth harmonic number.

Every solvable configuration of the Fifteen puzzle can be solved in no more than 43 multi-tile moves (i.e. when moving two or three tiles at once is counted as one move).

Science
The chemical element with the atomic number 43 is technetium. It has the lowest atomic number of any element that does not possess stable isotopes.

Music
The number of notes in Harry Partch's 43-tone scale of just intonation.

Literature

 * "Number 43", in Sonnets from the Portuguese (1850), is one of Elizabeth Barrett Browning's most famous poems.
 * 43 is the smallest number that is not mentioned in the Bible.

Mysticism
43 is the number of triangles inside the Sri Yantra.