21 = 1 − 23 − 45 + 6 − 7 + 89
21 = 9 + 87 − 6 − 5 − 43 − 21
21 = 0^5 + 1^8 − 2^7 − 3^9 + 4^3 + 5^6 + 6^2 + 7^0 + 8^4 + 9^1
21 = 2 + 2 + 17 = 3 + 5 + 13 = 3 + 7 + 11 = 5 + 5 + 11 = 7 + 7 + 7 (Sum of Primes)
21 = 1^2 + 2^2 + 4^2
221 - 1 = 7 × 7 × 127 × 337
21 = 3 x 7, F8 8th Fibonacci number
21 is the 5th Motzkin number.
21 = binomial(6 + 1, 2) is the 6th triangular number.
21 = 11 + 11 − 1
= 22 − 2/2
= 3 × (3 + 3) + 3
= 4 + 4 × 4 + 4/4 = 4!− 4 + 4 ÷ 4 = (44 − √4) ÷ √4
= 5 + 5 + 55/5
= 6 × (6 × 6 + 6)/(6 + 6)
= 7 + 7 + 7
= (88 + 88 − 8)/8
= 9 + (99 + 9)/9
1021 - 1 = 3 * 3 * 3 * 37 * 43 * 239 * 1933 * 4649 * 10838689
Moser-de Bruijn sequence: sums of distinct powers of 4
Numbers k such that (35*10^k - 11)/3 is prime
Number of prime knots with 8 crossings.
Number of Integer partitions of 21: 792
Number k such that k^2 + 2 is prime (443)
Jacobsthal number: a(n) = a(n-1) + 2*a(n-2), with a(0) = 0, a(1) = 1; also a(n) = nearest integer to 2^n/3. n = 6
Semiprime (Product of 2 Primes)
Semiprime s such that s-/+2 are primes.
Factors: 1, 3, 7, 21
Representations, Binary to Hexadecimal:
10101_2
210_3
111_4
41_5
33_6
30_7
25_8
23_9
1a_11
19_12
18_13
17_14
16_15
15_16
Undergraduate Texts in Mathematics
Graduate Studies in Mathematics
