21

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21 < 20 | 22 >
Numbers 0 - 99
0	1	2	3	4	5	6	7	8	9
10	11	12	13	14	15	16	17	18	19
20	21	22	23	24	25	26	27	28	29
30	31	32	33	34	35	36	37	38	39
40	41	42	43	44	45	46	47	48	49
50	51	52	53	54	55	56	57	58	59
60	61	62	63	64	65	66	67	68	69
70	71	72	73	74	75	76	77	78	79
80	81	82	83	84	85	86	87	88	89
90	91	92	93	94	95	96	97	98	99

21 (twenty-one) is a positive integer following 20 and preceding 22. Its ordinal form is written "21st" or "twenty-first". It can be also called long score.

Properties

• 21 is the smallest amounts of different size of squares to fill a larger square.
• 21 is an odd composite number.
• 21 is the 6th triangular number.
• 21 is deficient.

In googology

• In Greek-based number-naming systems, 21 is associated with prefix "henicosa-", and with prefix "unviginti-" in Latin systems.
• It is equal to S(3) with the S function, a variant of the Busy beaver function. This means 21 is the maximum finite number of steps that can be performed using a 3-state, 2-color Turing machine.
• DeepLineMadom calls the number zeroogoldeciplex and unoo-endecol.

Googological functions returning 21

• weak Goodstein function: \(g(4)=21\)
• Maximum shifts function: \(S(3)=21\)

In other languages

In languages such as Dutch and German, the ones are put before the tens; the resulting names are eenentwintig and einundzwanzig, respectively. There is a German club requesting an end to this practice.