40,000

40,000 (forty thousand) is the natural number that comes after 39,999 and before 40,001. It is the square of 200.

40001 to 40999

 * 40320 = smallest factorial (8!) that is not a highly composite number
 * 40425 = square pyramidal number
 * 40585 = largest factorion
 * 40678 = pentagonal pyramidal number
 * 40755 = Triangular number, Pentagonal number, Hexagonal number, 390-gonal, 4077-gonal, and 13586-gonal number.
 * 40804 = palindromic square

41000 to 41999

 * 41041 = Carmichael number
 * 41472 = 3-smooth number, number of reduced trees with 24 nodes
 * 41586 = Large Schröder number
 * 41616 = triangular square number
 * 41835 = Motzkin number
 * 41841 = 1/41841 = 0.0000239 is a repeating decimal with period 7

42000 to 42999

 * 42680 = octahedral number
 * 42875 = 353
 * 42925 = square pyramidal number

43000 to 43999

 * 43261 = Markov number
 * 43380 = number of nets of a dodecahedron
 * 43390 = number of primes $$\leq 2^{19}$$.
 * 43560 = pentagonal pyramidal number
 * 43691 = Wagstaff prime
 * 43777 = smallest member of a prime sextuplet

44000 to 44999

 * 44044 = palindrome of 79 after 6 iterations of the "reverse and add" iterative process
 * 44100 = sum of the cubes of the first 20 positive integers 44,100 Hz is a common sampling frequency in digital audio (and is the standard for compact discs).
 * 44444 = repdigit
 * 44583 = number of partitions of 41
 * 44721 = smallest positive integer such that the expression $1⁄n$ − $1⁄n + 2$ ≤ 10−9
 * 44724 = maximum number of days in which a human being has been verified to live (Jeanne Calment).
 * 44944 = palindromic square

45000 to 45999

 * 45360 = highly composite number; smallest number with exactly 100 factors (including one and itself)

46000 to 46999

 * 46080 = double factorial of 12
 * 46233 = sum of the first eight factorials
 * 46249 = 2nd number that can be written as $$a^2+b!$$ in 3 ways
 * 46368 = Fibonacci number
 * 46656 = 216$2$ = 36$3$ = 66, 3-smooth number
 * 46657 = Carmichael number

47000 to 47999

 * 47058 = primary pseudoperfect number
 * 47160 = 10-th derivative of xx at x=1
 * 47321/33461 ≈ √2

48000 to 48999

 * 48629 = number of trees with 17 unlabeled nodes
 * 48734 = number of 22-bead necklaces (turning over is allowed) where complements are equivalent

49000 to 49999

 * 49151 = Woodall number
 * 49152 = 3-smooth number
 * 49726 = pentagonal pyramidal number
 * 49940 = number of 21-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed

Primes
There are 930 prime numbers between 40000 and 50000.