friendly number

{{Short description|Two or more natural numbers with a common abundancy index}}

{{Distinguish|Amicable number}}

In number theory, friendly numbers are two or more natural numbers with a common abundancy index, the ratio between the sum of divisors of a number and the number itself. Two numbers with the same "abundancy" form a friendly pair; n numbers with the same abundancy form a friendly n-tuple.

Being mutually friendly is an equivalence relation, and thus induces a partition of the positive naturals into clubs (equivalence classes) of mutually friendly numbers.

A number that is not part of any friendly pair is called solitary.

The abundancy index of n is the rational number σ(n) / n, in which σ denotes the sum of divisors function. A number n is a friendly number if there exists mn such that σ(m) / m = σ(n) / n. Abundancy is not the same as abundance, which is defined as σ(n) − 2n.

Abundancy may also be expressed as \sigma_{-1}(n) where \sigma_k denotes a divisor function with \sigma_{k}(n) equal to the sum of the k-th powers of the divisors of n.

The numbers 1 through 5 are all solitary. The smallest friendly number is 6, forming for example, the friendly pair 6 and 28 with abundancy σ(6) / 6 = (1+2+3+6) / 6 = 2, the same as σ(28) / 28 = (1+2+4+7+14+28) / 28 = 2. The shared value 2 is an integer in this case but not in many other cases. Numbers with abundancy 2 are also known as perfect numbers. There are several unsolved problems related to the friendly numbers.

In spite of the similarity in name, there is no specific relationship between the friendly numbers and the amicable numbers or the sociable numbers, although the definitions of the latter two also involve the divisor function.

Examples

As another example, 30 and 140 form a friendly pair, because 30 and 140 have the same abundancy:{{cite web|url=https://tomrocksmaths.com/2023/05/10/numbers-with-cool-names-amicable-sociable-friendly/ |title=Numbers with Cool Names: Amicable, Sociable, Friendly|date=10 May 2023 |access-date=26 July 2023}}

: \dfrac{\sigma(30)}{30} = \dfrac{1+2+3+5+6+10+15+30}{30} =\dfrac{72}{30} = \dfrac{12}{5}

: \dfrac{\sigma(140)}{140} = \dfrac{1+2+4+5+7+10+14+20+28+35+70+140}{140} = \dfrac{336}{140} = \dfrac{12}{5}.

The numbers 2480, 6200 and 40640 are also members of this club, as they each have an abundancy equal to 12/5.

For an example of odd numbers being friendly, consider 135 and 819 (abundancy 16/9 (deficient)). There are also cases of even numbers being friendly to odd numbers, such as 42, 3472, 56896, ... {{OEIS|A347169}} and 544635 (abundancy of 16/7). The odd friend may be less than the even one, as in 84729645 and 155315394 (abundancy of 896/351), or in 6517665, 14705145 and 2746713837618 (abundancy of 64/27).

A square number can be friendly, for instance both 693479556 (the square of 26334) and 8640 have abundancy 127/36 (this example is credited to Dean Hickerson).

= Status for small ''n'' =

In the table below, blue numbers are proven friendly {{OEIS|id=A074902}}, red numbers are proven solitary {{OEIS|id=A095739}}, numbers n such that n and \sigma(n) are coprime {{OEIS|id=A014567}} are left uncolored, though they are known to be solitary. Other numbers have unknown status and are yellow.

File:The sum of an integer's unique factors.png

File:The "friendly-number" index of integers up to 2000.png

class="wikitable"
n\sigma(n)\frac{\sigma(n)}{n}
111
233/2
344/3
477/4
566/5
style="background: blue; color: white;"

| 6

122
788/7
81515/8
91313/9
style="background: yellow; color: black;"

| 10

189/5
111212/11
style="background: blue; color: white;"

| 12

287/3
131414/13
style="background: yellow; color: black;"

| 14

2412/7
style="background: yellow; color: black;"

| 15

248/5
163131/16
171818/17
style="background: red; color: white;"

| 18

3913/6
192020/19
style="background: yellow; color: black;"

| 20

4221/10
213232/21
style="background: yellow; color: black;"

| 22

3618/11
232424/23
style="background: blue; color: white;"

| 24

605/2
253131/25
style="background: yellow; color: black;"

| 26

4221/13
274040/27
style="background: blue; color: white;"

| 28

562
293030/29
style="background: blue; color: white;"

| 30

7212/5
313232/31
326363/32
style="background: yellow; color: black;"

| 33

4816/11
style="background: yellow; color: black;"

| 34

5427/17
354848/35
369191/36

class="wikitable"
n\sigma(n)\frac{\sigma(n)}{n}
373838/37
style="background: yellow; color: black;"

| 38

6030/19
395656/39
style="background: blue; color: white;"

| 40

909/4
414242/41
style="background: blue; color: white;"

| 42

9616/7
434444/43
style="background: yellow; color: black;"

| 44

8421/11
style="background: red; color: white;"

| 45

7826/15
style="background: yellow; color: black;"

| 46

7236/23
474848/47
style="background: red; color: white;"

| 48

12431/12
495757/49
509393/50
style="background: yellow; color: black;"

| 51

7224/17
style="background: red; color: white;"

| 52

9849/26
535454/53
style="background: yellow; color: black;"

| 54

12020/9
557272/55
style="background: blue; color: white;"

| 56

12015/7
578080/57
style="background: yellow; color: black;"

| 58

9045/29
596060/59
style="background: blue; color: white;"

| 60

16814/5
616262/61
style="background: yellow; color: black;"

| 62

9648/31
63104104/63
64127127/64
658484/65
style="background: blue; color: white;"

| 66

14424/11
676868/67
style="background: yellow; color: black;"

| 68

12663/34
style="background: yellow; color: black;"

| 69

9632/23
style="background: yellow; color: black;"

| 70

14472/35
717272/71
style="background: yellow; color: black;"

| 72

19565/24

class="wikitable"
n\sigma(n)\frac{\sigma(n)}{n}
737474/73
style="background: yellow; color: black;"

| 74

11457/37
75124124/75
style="background: yellow; color: black;"

| 76

14035/19
779696/77
style="background: blue; color: white;"

| 78

16828/13
798080/79
style="background: blue; color: white;"

| 80

18693/40
81121121/81
style="background: yellow; color: black;"

| 82

12663/41
838484/83
style="background: blue; color: white;"

| 84

2248/3
85108108/85
style="background: yellow; color: black;"

| 86

13266/43
style="background: yellow; color: black;"

| 87

12040/29
style="background: yellow; color: black;"

| 88

18045/22
899090/89
style="background: yellow; color: black;"

| 90

23413/5
style="background: yellow; color: black;"

| 91

11216/13
style="background: yellow; color: black;"

| 92

16842/23
93128128/93
style="background: yellow; color: black;"

| 94

14472/47
style="background: yellow; color: black;"

| 95

12024/19
style="background: blue; color: white;"

| 96

25221/8
979898/97
98171171/98
style="background: yellow; color: black;"

| 99

15652/33
100217217/100
101102102/101
style="background: blue; color: white;"

| 102

21636/17
103104104/103
style="background: yellow; color: black;"

| 104

210105/52
style="background: yellow; color: black;"

| 105

19264/35
style="background: yellow; color: black;"

| 106

16281/53
107108108/107
style="background: blue; color: white;"

| 108

28070/27

class="wikitable"
n\sigma(n)\frac{\sigma(n)}{n}
109110110/109
style="background: yellow; color: black;"

| 110

216108/55
111152152/111
style="background: yellow; color: black;"

| 112

24831/14
113114114/113
style="background: blue; color: white;"

| 114

24040/19
115144144/115
style="background: yellow; color: black;"

| 116

210105/58
style="background: yellow; color: black;"

| 117

18214/9
style="background: yellow; color: black;"

| 118

18090/59
119144144/119
style="background: blue; color: white;"

| 120

3603
121133133/121
style="background: yellow; color: black;"

| 122

18693/61
style="background: yellow; color: black;"

| 123

16856/41
style="background: yellow; color: black;"

| 124

22456/31
125156156/125
style="background: yellow; color: black;"

| 126

31252/21
127128128/127
128255255/128
129176176/129
style="background: yellow; color: black;"

| 130

252126/65
131132132/131
style="background: blue; color: white;"

| 132

33628/11
133160160/133
style="background: yellow; color: black;"

| 134

204102/67
style="background: blue; color: white;"

| 135

24016/9
style="background: red; color: white;"

| 136

270135/68
137138138/137
style="background: blue; color: white;"

| 138

28848/23
139140140/139
style="background: blue; color: white;"

| 140

33612/5
style="background: yellow; color: black;"

| 141

19264/47
style="background: yellow; color: black;"

| 142

216108/71
143168168/143
144403403/144

Solitary numbers

A number that belongs to a singleton club, because no other number is friendly with it, is a solitary number. All prime numbers are known to be solitary, as are powers of prime numbers. More generally, if the numbers n and σ(n) are coprime – meaning that the greatest common divisor of these numbers is 1, so that σ(n)/n is an irreducible fraction – then the number n is solitary {{OEIS|id=A014567}}. For a prime number p we have σ(p) = p + 1, which is co-prime with p.

No general method is known for determining whether a number is friendly or solitary.

Is 10 a solitary number?

The smallest number whose classification is unknown is 10; it is conjectured to be solitary. If it is not, its smallest friend is at least 10^{30}. J. Ward {{Citation |last=Ward |first=Jeffrey |title=Does Ten Have a Friend? |date=2008-06-06 |volume= |pages= |publisher= |arxiv=0806.1001 }}proved that any positive integer n other than 10 with abundancy index \frac{9}{5} must be a square with at least six distinct prime factors, the smallest being 5. Further, at least one of the prime factors must be congruent to 1 modulo 3 and appear with an exponent congruent to 2 modulo 6 in the prime power factorization of n. HR (Maya) Thackeray {{Cite journal |last=Thackeray |first=Henry (Maya) Robert |date=2024-05-01 |title=Each friend of 10 has at least 10 nonidentical prime factors |url=https://pdf.sciencedirectassets.com/273071/1-s2.0-S0019357724X00038/1-s2.0-S0019357724000430/main.pdf?X-Amz-Security-Token=IQoJb3JpZ2luX2VjEKv%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaCXVzLWVhc3QtMSJIMEYCIQD6T524dFKrWTMUXr1TYKhJxNP18h%2BjkI6sWLJizzaBNgIhAIx8XqNGsrw7FUTBfAtxa6U4hRP2M2ULj02V1HIu%2FArJKrMFCDQQBRoMMDU5MDAzNTQ2ODY1IgzAlCKZPIBATzABEWAqkAX06YA4tCle9kfAtYmoyL4WmbtMF7X6D%2BE6%2FFnnC%2FAjIrKmJfF2wEyLoMUVBipv%2BsIa7tsod7eQ5Coodeo3tjYstaU92WGrXwF8cVlHScG8ieWyAJeHGw%2BuxdMubf9Qn8KbzFE%2FWTbaRxmJ%2B8LxJLi3bzASi2rJ%2Ft9Zt1cQYurMAUf7187q16ftff49bpzws1c5zftKl3ntvpqOwd4m9eWmWXRLEYOP1fNnh1YKWi73TjOFyEucL1Wuhy7vVlmfvSrRIvLog6QjYZFcUqkEUUQ9bXmrnTSrdPNqT6pgBqVRHNrH9GnriZPKT5rWEqkLpcF0%2FkOL%2B6XE6BlPTbLBsTKqHVY1cAd%2BSjBsmzkzIUV%2BqLhJjz6evhABMzFSh2dK9ekWac%2F2%2BjNIxVFPH%2FFTLc6dS8kGfhO2OFjPln64Qudd%2FrBq7MK%2FdW4uQBzl5rujHYaU7EwMCcTy1B5dl%2FPdFq%2BkOZL5AoanShjRWMFWosX1v8X8vUxeSuFjpouf7aybLhvh5Ob8c472ZpWC48%2BQdINUiHHeYV5Rx9IbL2O3XGNRJHB6mkeWGwXCn%2BjjE54pRN0EIZeZ1w37rUuRArHT3%2BljntRG3kNEklVN5itetzTZfSLVfbQw4Dg48ECx%2FwTY7HyAUOXUUyHwGUnWHwwa8hjLCNbpHjGy4ECZsFvbq22dtpGxIKNC4X52wcpS%2B6E73Y4VssbFbsU2Z4lkXsroGHM3%2FPiUwpU%2FVYjhu7nyrmCmkGB7aHkcyzOWlnAxa19IiZ0DxCLELFq7l22ndV%2Bf62CSNbOfB%2BKsVpQmhEvLsZgMCWn0AzwrzTiLwFV%2FxmhY0F2oD7icZJmGfH5WxT3w1ngXB4VuNWdZC6v5Mjz7JY6g6jDxx%2Fq%2FBjqwAaa5MSdDchaHjQzsBcHaIeTe4u9zdDvo%2BAdWSobxYD8XhKv5pAL2%2FyY5n3iMLNkS%2BWecux13gUMfGcrwpIflYOlO26o37%2F9ZBb5SkCKuF1r59YFQ2NhGTxMJJ6xVPi5iyDJsyotYjY7RX%2FfnYdvHTlYUQc320jzYYj%2Bg8lwe7kTnNwwin7%2FyGQSBYz%2FNtBMdt8DuK3l0N7N5ng64W%2BkSEvVrw05PSouwQd36GLZMMW0c&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20250415T183202Z&X-Amz-SignedHeaders=host&X-Amz-Expires=300&X-Amz-Credential=ASIAQ3PHCVTYUKT3QJIX%2F20250415%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Signature=463aa5f209b250215e7669d278171572a4d11442bbcc80e47192632dd4fb47c1&hash=f774be43e8def6cce9901505a37bf3995a28ae6a0de422f83db74c6d70311f8b&host=68042c943591013ac2b2430a89b270f6af2c76d8dfd086a07176afe7c76c2c61&pii=S0019357724000430&tid=spdf-8b8b8869-340c-4896-a66e-105cc0c9255c&sid=4e8a662445e83044b648af277674fac1cd80gxrqb&type=client&tsoh=d3d3LnNjaWVuY2VkaXJlY3QuY29t&rh=d3d3LnNjaWVuY2VkaXJlY3QuY29t&ua=1308595751075f57055256&rr=930d85f51c0879d5&cc=in&kca=eyJrZXkiOiJEVXpOYStPR0lBbmpKS1JxY1hyQVoreXQwdlJWR05aQzkyQW9nN1lYN2hBYTlCY1ZIMlZoVlp4MkU3OHFlWlIwdnJ1MFUxYVlQMWdqYlhqL2t1eWZYbmJ0QUNJSGREMlhWU2ZWZTRta2pwR1RRaDU1WlRBYk9PTDRRdmlybkdJWTRlNHlRUjU5dzZNTWhJRG94UVZiZWw5VTVpUlRhUHVNMHBPaWFvekl4ZjN5eWlqSSIsIml2IjoiMzQ5MTE1OTE0MTBmNTIxNjk3MmEyY2Y2NDAyMDc1MzIifQ==_1744741939807 |url-status= |journal=Indagationes Mathematicae |volume=35 |issue=3 |pages=595–607 |arxiv=2310.15900 |doi=10.1016/j.indag.2024.04.011 |issn=0019-3577}} applied methods from Nielsen’s {{Cite journal |last=Nielsen |first=Pace P. |date=2007-10-01 |title=Odd perfect numbers have at least nine distinct prime factors |url=https://www.ams.org/journals/mcom/2007-76-260/S0025-5718-07-01990-4/S0025-5718-07-01990-4.pdf |journal=Mathematics of Computation |volume=76 |issue=260 |pages=2109–2127 |arxiv=math/0602485 |doi=10.1090/S0025-5718-07-01990-4|bibcode=2007MaCom..76.2109N }} to show that each friend of 10 has at least 10 nonidentical prime factors. Sourav Mandal and Sagar Mandal {{Cite journal |last1=Mandal |first1=Sourav |last2=Mandal |first2=Sagar |date=2025-02-01 |title=Upper Bounds for the Prime Divisors of Friends of 10 |url=https://link.springer.com/content/pdf/10.1007/s12045-025-1747-8.pdf |journal=Resonance |language=en |volume=30 |issue=2 |pages=263–275 |arxiv=2404.05771 |doi=10.1007/s12045-025-1747-8 |issn=0973-712X}} proved that if n is a friend of 10 and if q_2,q_3, q_4 are the second, third, fourth smallest prime divisors of n respectively then

7\leq q_2<\left \lceil \frac{7\omega( n )}{3} \right \rceil\biggl(\log\left \lceil \frac{7\omega( n )}{3} \right \rceil+2\log \log \left \lceil \frac{7\omega(n)}{ 3 } \right \rceil\biggr),

11\leq q_3<\left \lceil \frac{180\omega( n )}{41} \right \rceil\biggl(\log\left \lceil \frac{180\omega( n )}{41} \right \rceil+2\log \log \left \lceil \frac{180\omega(n)}{ 41 } \right \rceil\biggr),

13\leq q_4<\left \lceil \frac{390\omega( n )}{47} \right \rceil\biggl(\log\left \lceil \frac{390\omega( n )}{47} \right \rceil+2\log \log \left \lceil \frac{390\omega(n)}{47} \right \rceil\biggr) ,

where \omega(n) is the number of distinct prime divisors of n and \left \lceil \right \rceil is the ceiling function. S. Mandal {{Cite journal |last=Mandal |first=Sagar |date=2025-04-12 |title=Exploring the Relationships Between the Divisors of Friends of 10 |url=https://zenodo.org/records/15206287/files/CMS-21-32.pdf?download=1 |journal=News Bulletin of Calcutta Mathematical Society |volume=48 |issue=1–3 |pages=21–32 |arxiv=2504.08295 |doi=10.5281/zenodo.15206286 |issn=0970-8596}} proved that not all half of the exponents of the prime divisors of a friend of 10 are congruent to 1 modulo 3. Further, he proved that if n= 5^{2a}\cdot Q^2 (Q is an odd positive integer coprime to 15 ) is a friend of 10, then \sigma(5^{2a})+\sigma(Q^{2}) is congruent to 6 modulo 8 if and only if a is even, and \sigma(5^{2a})+\sigma(Q^{2}) is congruent to 2 modulo 8 if and only if a is odd. In addition, he established that n> \frac{25}{81}\cdot \prod_{i=1}^{\omega(n)} (2a_i+1)^2, in particular n>625\cdot 9^{\omega(n)-3} by setting Q=\prod_{i=2}^{\omega(n)} p_i^{a_i} and a=a_1, where p_i are prime numbers.

Small numbers with a relatively large smallest friend do exist: for instance, 24 is friendly, with its smallest friend 91,963,648.{{cite web|last1=Cemra|first1=Jason|title=10 Solitary Check|url=https://github.com/CemraJC/solidarity|website=Github/CemraJC/Solidarity|date=23 July 2022 }}{{cite encyclopedia |url=https://oeis.org/A074902 |title=OEIS sequence A074902 |encyclopedia=On-Line Encyclopedia of Integer Sequences |access-date=10 July 2020}}

Large clubs

It is an open problem whether there are infinitely large clubs of mutually friendly numbers. The perfect numbers form a club, and it is conjectured that there are infinitely many perfect numbers (at least as many as there are Mersenne primes), but no proof is known. There are clubs with more known members: in particular, those formed by multiply perfect numbers, which are numbers whose abundancy is an integer. Although some are known to be quite large, clubs of multiply perfect numbers (excluding the perfect numbers themselves) are conjectured to be finite.

Asymptotic density

Every pair a, b of friendly numbers gives rise to a positive proportion of all natural numbers being friendly (but in different clubs), by considering pairs na, nb for multipliers n with gcd(n, ab) = 1. For example, the "primitive" friendly pair 6 and 28 gives rise to friendly pairs 6n and 28n for all n that are congruent to 1, 5, 11, 13, 17, 19, 23, 25, 29, 31, 37, or 41 modulo 42.{{cite journal |jstor = 2318325|last1 = Anderson|first1 = C. W.|last2 = Hickerson|first2 = Dean|last3 = Greening|first3 = M. G.|title = 6020|journal = The American Mathematical Monthly|year = 1977|volume = 84|issue = 1|pages = 65–66|doi = 10.2307/2318325}}

This shows that the natural density of the friendly numbers (if it exists) is positive.

Anderson and Hickerson proposed that the density should in fact be 1 (or equivalently that the density of the solitary numbers should be 0). According to the MathWorld article on Solitary Number (see References section below), this conjecture has not been resolved, although Pomerance thought at one point he had disproved it.

Notes

References

  • Grime, James. [https://www.numberphile.com/videos/a-video-about-the-number-10 A video about the number 10]. Numberphile.
  • {{MathWorld |urlname=FriendlyNumber |title=Friendly Number}}
  • {{MathWorld |urlname=FriendlyPair |title=Friendly Pair}}
  • {{MathWorld |urlname=SolitaryNumber |title=Solitary Number}}
  • {{MathWorld |urlname=Abundancy |title=Abundancy}}

{{Divisor classes}}

{{Classes of natural numbers}}

Category:Divisor function

Category:Integer sequences

Category:Unsolved problems in number theory