Multiply perfect number

Demonstration, with Cuisenaire rods, of the Template:Nowrap of the number 6

In mathematics, a multiply perfect number (also called multiperfect number or pluperfect number) is a generalization of a perfect number.

For a given natural number k, a number n is called Template:Nowrap (or Template:Nowrap perfect) if the sum of all positive divisors of n (the divisor function, σ(n)) is equal to kn; a number is thus perfect if and only if it is Template:Nowrap. A number that is Template:Nowrap for a certain k is called a multiply perfect number. As of 2014, Template:Nowrap numbers are known for each value of k up to 11.

It is unknown whether there are any odd multiply perfect numbers other than 1. The first few multiply perfect numbers are:

1, 6, 28, 120, 496, 672, 8128, 30240, 32760, 523776, 2178540, 23569920, 33550336, 45532800, 142990848, 459818240, ... Template:OEIS.

Example

The sum of the divisors of 120 is

1 + 2 + 3 + 4 + 5 + 6 + 8 + 10 + 12 + 15 + 20 + 24 + 30 + 40 + 60 + 120 = 360

which is 3 × 120. Therefore 120 is a Template:Nowrap number.

Smallest known k-perfect numbers

The following table gives an overview of the smallest known Template:Nowrap numbers for k ≤ 11 Template:OEIS:

k	Smallest k-perfect number	Factors	Found by
1	1		ancient
2	6	2 × 3	ancient
3	120	23 × 3 × 5	ancient
4	30240	25 × 33 × 5 × 7	René Descartes, circa 1638
5	14182439040	27 × 34 × 5 × 7 × 112 × 17 × 19	René Descartes, circa 1638
6	154345556085770649600 (21 digits)	215 × 35 × 52 × 72 × 11 × 13 × 17 × 19 × 31 × 43 × 257	Robert Daniel Carmichael, 1907
7	141310897947438348259849...523264343544818565120000 (57 digits)	232 × 311 × 54 × 75 × 112 × 132 × 17 × 193 × 23 × 31 × 37 × 43 × 61 × 71 × 73 × 89 × 181 × 2141 × 599479	TE Mason, 1911
8	826809968707776137289924...057256213348352000000000 (133 digits)	262 × 315 × 59 × 77 × 113 × 133 × 172 × 19 × 23 × 29 × 312 × 37 × 41 × 43 × 53 × 612 × 712 × 73 × 83 × 89 × 972 × 127 × 193 × 283 × 307 × 317 × 331 × 337 × 487 × 5212 × 601 × 1201 × 1279 × 2557 × 3169 × 5113 × 92737 × 649657	Stephen F. Gretton, 1990
9	561308081837371589999987...415685343739904000000000 (287 digits)	2104 × 343 × 59 × 712 × 116 × 134 × 17 × 194 × 232 × 29 × 314 × 373 × 412 × 432 × 472 × 53 × 59 × 61 × 67 × 713 × 73 × 792 × 83 × 89 × 97 × 1032 × 107 × 127 × 1312 × 1372 × 1512 × 191 × 211 × 241 × 331 × 337 × 431 × 521 × 547 × 631 × 661 × 683 × 709 × 911 × 1093 × 1301 × 1723 × 2521 × 3067 × 3571 × 3851 × 5501 × 6829 × 6911 × 8647 × 17293 × 17351 × 29191 × 30941 × 45319 × 106681 × 110563 × 122921 × 152041 × 570461 × 16148168401	Fred Helenius, 1995
10	448565429898310924320164...000000000000000000000000 (639 digits)	2175 × 369 × 529 × 718 × 1119 × 138 × 179 × 197 × 239 × 293 × 318 × 372 × 414 × 434 × 474 × 533 × 59 × 615 × 674 × 714 × 732 × 79 × 83 × 89 × 97 × 1013 × 1032 × 1072 × 109 × 113 × 1272 × 1312 × 139 × 149 × 151 × 163 × 179 × 1812 × 191 × 197 × 199 × 2113 × 223 × 239 × 257 × 271 × 281 × 307 × 331 × 337 × 3532 × 367 × 373 × 397 × 419 × 421 × 521 × 523 × 5472 × 613 × 683 × 761 × 827 × 971 × 991 × 1093 × 1741 × 1801 × 2113 × 2221 × 2237 × 2437 × 2551 × 2851 × 3221 × 3571 × 3637 × 3833 × 4339 × 5101 × 5419 × 6577 × 6709 × 7621 × 7699 × 8269 × 8647 × 11093 × 13421 × 13441 × 14621 × 17293 × 26417 × 26881 × 31723 × 44371 × 81343 × 88741 × 114577 × 160967 × 189799 × 229153 × 292561 × 579281 × 581173 × 583367 × 1609669 × 3500201 × 119782433 × 212601841 × 2664097031 × 2931542417 × 43872038849 × 374857981681 × 4534166740403	George Woltman, 2013
11	251850413483992918774837...000000000000000000000000 (1907 digits)	2468 × 3140 × 566 × 749 × 1140 × 1331 × 1711 × 1912 × 239 × 297 × 3111 × 378 × 415 × 433 × 473 × 534 × 593 × 612 × 674 × 714 × 733 × 79 × 832 × 89 × 974 × 1014 × 1033 × 1093 × 1132 × 1273 × 1313 × 1372 × 1392 × 1492 × 151 × 1572 × 163 × 167 × 173 × 181 × 191 × 1932 × 197 × 199 × 2113 × 223 × 227 × 2292 × 239 × 251 × 257 × 263 × 2693 × 271 × 2812 × 293 × 3073 × 313 × 317 × 331 × 347 × 349 × 367 × 373 × 397 × 401 × 419 × 421 × 431 × 4432 × 449 × 457 × 461 × 467 × 491 × 4992 × 541 × 547 × 569 × 571 × 599 × 607 × 613 × 647 × 691 × 701 × 719 × 727 × 761 × 827 × 853 × 937 × 967 × 991 × 997 × 1013 × 1061 × 1087 × 1171 × 1213 × 1223 × 1231 × 1279 × 1381 × 1399 × 1433 × 1609 × 1613 × 1619 × 1723 × 1741 × 1783 × 1873 × 1933 × 1979 × 2081 × 2089 × 2221 × 2357 × 2551 × 2657 × 2671 × 2749 × 2791 × 2801 × 2803 × 3331 × 3433 × 4051 × 4177 × 4231 × 5581 × 5653 × 5839 × 6661 × 7237 × 7699 × 8081 × 8101 × 8269 × 8581 × 8941 × 10501 × 11833 × 12583 × 12941 × 13441 × 14281 × 15053 × 17929 × 19181 × 20809 × 21997 × 23063 × 23971 × 26399 × 26881 × 27061 × 28099 × 29251 × 32051 × 32059 × 32323 × 33347 × 33637 × 36373 × 38197 × 41617 × 51853 × 62011 × 67927 × 73547 × 77081 × 83233 × 92251 × 93253 × 124021 × 133387 × 141311 × 175433 × 248041 × 256471 × 262321 × 292561 × 338753 × 353641 × 441281 × 449653 × 509221 × 511801 × 540079 × 639083 × 696607 × 746023 × 922561 × 1095551 × 1401943 × 1412753 × 1428127 × 1984327 × 2556331 × 5112661 × 5714803 × 7450297 × 8334721 × 10715147 × 14091139 × 14092193 × 18739907 × 19270249 × 29866451 × 96656723 × 133338869 × 193707721 × 283763713 × 407865361 × 700116563 × 795217607 × 3035864933 × 3336809191 × 35061928679 × 143881112839 × 161969595577 × 287762225677 × 761838257287 × 840139875599 × 2031161085853 × 2454335007529 × 2765759031089 × 31280679788951 × 75364676329903 × 901563572369231 × 2169378653672701 × 4764764439424783 × 70321958644800017 × 79787519018560501 × 702022478271339803 × 1839633098314450447 × 165301473942399079669 × 604088623657497125653141 × 160014034995323841360748039 × 25922273669242462300441182317 × 15428152323948966909689390436420781 × 420391294797275951862132367930818883361 × 23735410086474640244277823338130677687887 × 628683935022908831926019116410056880219316806841500141982334538232031397827230330241	George Woltman, 2001

Properties

It can be proven that:

• For a given prime number p, if n is Template:Nowrap and p does not divide n, then pn is Template:Nowrap. This implies that an integer n is a Template:Nowrap number divisible by 2 but not by 4, if and only if n/2 is an odd perfect number, of which none are known.
• If 3n is Template:Nowrap and 3 does not divide n, then n is Template:Nowrap.

Odd multiply perfect numbers

It is unknown whether there are any odd multiply perfect numbers other than 1. However if an odd Template:Nowrap number n exists where k > 2, then it must satisfy the following conditions:

• The largest prime factor is ≥ 100129
• The second largest prime factor is ≥ 1009
• The third largest prime factor is ≥ 101

Bounds

In little-o notation, the number of multiply perfect numbers less than x is $o(x^{\epsilon })$ for all ε > 0.

The number of k-perfect numbers n for n ≤ x is less than $c{x}^{c^{\prime }\log \log \log x/\log \log x}$, where c and c' are constants independent of k.

Under the assumption of the Riemann hypothesis, the following inequality is true for all Template:Nowrap numbers n, where k > 3

$\log \log n>k\cdot e^{-\gamma }$

where $\gamma$ is Euler's gamma constant. This can be proven using Robin's theorem.

The number of divisors τ(n) of a Template:Nowrap number n satisfies the inequality

$\tau (n)>e^{k-\gamma }.$

The number of distinct prime factors ω(n) of n satisfies

$\omega (n)\ge k^2-1.$

If the distinct prime factors of n are $p_1,p_2,\dots ,p_r$, then:

$r(\sqrt[r]{3/2}-1)<\sum _{i=1}^r\frac{1}{p_i}<r(1-\sqrt[r]{6/k^2}),\text{if }n\text{ is even}$

$r(\sqrt[3r]{k^2}-1)<\sum _{i=1}^r\frac{1}{p_i}<r(1-\sqrt[r]{8/(k{\pi }^2)}),\text{if }n\text{ is odd}$

Specific values of k

Perfect numbers

Template:Main A number n with σ(n) = 2n is perfect.

Triperfect numbers

A number n with σ(n) = 3n is triperfect. There are only six known triperfect numbers and these are believed to comprise all such numbers:

120, 672, 523776, 459818240, 1476304896, 51001180160 Template:OEIS

If there exists an odd perfect number m (a famous open problem) then 2m would be Template:Nowrap, since σ(2m) = σ(2) σ(m) = 3×2m. An odd triperfect number must be a square number exceeding 10⁷⁰ and have at least 12 distinct prime factors, the largest exceeding 10⁵.

Variations

Unitary multiply perfect numbers

A similar extension can be made for unitary perfect numbers. A positive integer n is called a unitary multi Template:Nowrap number if σ^*(n) = kn where σ^*(n) is the sum of its unitary divisors. (A divisor d of a number n is a unitary divisor if d and n/d share no common factors.).

A unitary multiply perfect number is simply a unitary multi Template:Nowrap number for some positive integer k. Equivalently, unitary multiply perfect numbers are those n for which n divides σ^*(n). A unitary multi Template:Nowrap number is naturally called a unitary perfect number. In the case k > 2, no example of a unitary multi Template:Nowrap number is yet known. It is known that if such a number exists, it must be even and greater than 10¹⁰² and must have more than forty four odd prime factors. This problem is probably very difficult to settle. The concept of unitary divisor was originally due to R. Vaidyanathaswamy (1931) who called such a divisor as block factor. The present terminology is due to E. Cohen (1960).

The first few unitary multiply perfect numbers are:

1, 6, 60, 90, 87360 Template:OEIS

Bi-unitary multiply perfect numbers

A positive integer n is called a bi-unitary multi Template:Nowrap number if σ^**(n) = kn where σ^**(n) is the sum of its bi-unitary divisors. This concept is due to Peter Hagis (1987). A bi-unitary multiply perfect number is simply a bi-unitary multi Template:Nowrap number for some positive integer k. Equivalently, bi-unitary multiply perfect numbers are those n for which n divides σ^**(n). A bi-unitary multi Template:Nowrap number is naturally called a bi-unitary perfect number, and a bi-unitary multi Template:Nowrap number is called a bi-unitary triperfect number.

A divisor d of a positive integer n is called a bi-unitary divisor of n if the greatest common unitary divisor (gcud) of d and n/d equals 1. This concept is due to D. Surynarayana (1972). The sum of the (positive) bi-unitary divisors of n is denoted by σ^**(n).

Peter Hagis (1987) proved that there are no odd bi-unitary multiperfect numbers other than 1. Haukkanen and Sitaramaiah (2020) found all bi-unitary triperfect numbers of the form 2^au where 1 ≤ a ≤ 6 and u is odd, and partially the case where a = 7. Further, they fixed completely the case a = 8.

The first few bi-unitary multiply perfect numbers are:

1, 6, 60, 90, 120, 672, 2160, 10080, 22848, 30240 Template:OEIS

References

Sources

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See also

• Hemiperfect number

External links

• The Multiply Perfect Numbers page
• The Prime Glossary: Multiply perfect numbers
• Template:YouTube