A030516 -id:A030516 - cp's OEIS frontend

A280346 Numbers with 79 divisors.

302231454903657293676544, 16423203268260658146231467800709255289, 3308722450212110699485634768279851414263248443603515625, 827269706064171159838078900184013751038269841857389464208009274449, 1692892739326831320764318961708001178036611459414853872137348292520966629744627081

Offset: 1

Omar E. Pol, Jan 01 2017

Also, 78th powers of primes.

More generally, the n-th number with p divisors is equal to the n-th prime raised to power p-1, where p is prime. In this case, p = 79.

			a(1) = 2^78, a(2) = 3^78, a(3) = 5^78, a(4) = 7^78, a(5) = 11^78.

OEIS Wiki, Index entries for number of divisors

Cf. A000005, A000040, A001248, A030514, A030516, A030629, A030631, A030635, A030637, A137486, A137492, A139571, A139572, A139573, A139574, A139575, A173533, A183062, A183085, A280298, A280299, A280301, A280347, A280349, A261700.

With[{p = 22}, Table[Prime[n]^(Prime@ p - 1), {n, 5}]] (* Michael De Vlieger, Jan 01 2017 *)

PARI
```
a(n)=prime(n)^78
```

a(n) = A000040(n)^(79-1) = A000040(n)^78.

A000005(a(n)) = 79.

A280347 Numbers with 83 divisors.

Original entry on oeis.org

4835703278458516698824704, 1330279464729113309844748891857449678409, 2067951531382569187178521730174907133914530277252197265625, 1986274564260074954771227439341817016242885890299592103563430267952049, 24785642596484137367310393918366845247634028377292875541962916350799472426091085092921

Offset: 1

Omar E. Pol, Jan 01 2017

Also, 82nd powers of primes.

More generally, the n-th number with p divisors is equal to the n-th prime raised to power p-1, where p is prime. In this case, p = 83.

			a(1) = 2^82, a(2) = 3^82, a(3) = 5^82, a(4) = 7^82, a(5) = 11^82.

OEIS Wiki, Index entries for number of divisors

Cf. A000005, A000040, A001248, A030514, A030516, A030629, A030631, A030635, A030637, A137486, A137492, A139571, A139572, A139573, A139574, A139575, A173533, A183062, A183085, A280298, A280299, A280301, A280346, A280349, A261700.

With[{p = 23}, Table[Prime[n]^(Prime@ p - 1), {n, 5}]] (* Michael De Vlieger, Jan 01 2017 *)

PARI
```
a(n)=prime(n)^82
```

a(n) = A000040(n)^(83-1) = A000040(n)^82.

A000005(a(n)) = 83.

A280349 Numbers with 89 divisors.

Original entry on oeis.org

309485009821345068724781056, 969773729787523602876821942164080815560161, 32311742677852643549664402033982923967414535582065582275390625, 233683216210633558353880137011125430143959282107856711392134007594290612801, 43909277783870034878569768760415886733743786946105343887995366053338664170638348798300219681

Offset: 1

Omar E. Pol, Jan 01 2017

Also, 88th powers of primes.

More generally, the n-th number with p divisors is equal to the n-th prime raised to power p-1, where p is prime. In this case, p = 89.

			a(1) = 2^88, a(2) = 3^88, a(3) = 5^88, a(4) = 7^88, a(5) = 11^88.

OEIS Wiki, Index entries for number of divisors

Cf. A000005, A000040, A001248, A030514, A030516, A030629, A030631, A030635, A030637, A137486, A137492, A139571, A139572, A139573, A139574, A139575, A173533, A183062, A183085, A280298, A280299, A280301, A280346, A280347, A261700.

With[{p = 24}, Table[Prime[n]^(Prime@ p - 1), {n, 5}]] (* Michael De Vlieger, Jan 01 2017 *)

PARI
```
a(n)=prime(n)^88
```

a(n) = A000040(n)^(89-1) = A000040(n)^88.

A000005(a(n)) = 89.

A122220 a(n) = (prime(n)^6 - prime(n)^2)/20.

Original entry on oeis.org

3, 36, 780, 5880, 88572, 241332, 1206864, 2352276, 7401768, 29741124, 44375136, 128286252, 237505128, 316068060, 538960656, 1108217916, 2109026508, 2576018532, 4522918884, 6405013944, 7566711048, 12154372464, 16347018324, 24849064152, 41648599776

Offset: 1

Artur Jasinski, Apr 16 2008

Cf. A138448, A030516, A001248, A138441, A030514.

A122220:=n->(ithprime(n)^6-ithprime(n)^2)/20: seq(A122220(n), n=1..40); # Wesley Ivan Hurt, Apr 14 2017

(#^6-#^2)/20&/@Prime[Range[30]] (* Harvey P. Dale, Jan 19 2011 *)

forprime(p=2,1e3,print1((p^6-p^2)/20", ")) \\ Charles R Greathouse IV, Jul 15 2011

From Elmo R. Oliveira, Jan 18 2023: (Start)
a(n) = (A030516(n) - A001248(n))/20.
a(n) = A138441(n)/10.
a(n) = (A001248(n) * (A030514(n) - 1))/20. (End)

A137489 Numbers with 26 divisors.

Original entry on oeis.org

12288, 20480, 28672, 45056, 53248, 69632, 77824, 94208, 118784, 126976, 151552, 167936, 176128, 192512, 217088, 241664, 249856, 274432, 290816, 299008, 323584, 339968, 364544, 397312, 413696, 421888, 438272, 446464, 462848, 520192, 536576

Offset: 1

R. J. Mathar, Apr 22 2008

nonn

Maple implementation: see A030513.

Numbers of the form p^25 (5th powers of A050997, subset of A010813) or p*q^12, where p and q are distinct primes. - R. J. Mathar, Mar 01 2010

T. D. Noe, Table of n, a(n) for n = 1..1000

Cf. A030513-A030516, A030626, A030627, A030634-A030638, A005179, A003680, A096932, A061286, A061283.

Select[Range[900000],DivisorSigma[0,#]==26&] (* Vladimir Joseph Stephan Orlovsky, May 05 2011 *)

is(n)=numdiv(n)==26 \\ Charles R Greathouse IV, Jun 19 2016

A000005(a(n))=26.

A336530 Number of triples of divisors d_i < d_j < d_k of n such that gcd(d_i, d_j, d_k) > 1.

Original entry on oeis.org

0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 5, 0, 0, 0, 4, 0, 5, 0, 5, 0, 0, 0, 23, 0, 0, 1, 5, 0, 12, 0, 10, 0, 0, 0, 36, 0, 0, 0, 23, 0, 12, 0, 5, 5, 0, 0, 62, 0, 5, 0, 5, 0, 23, 0, 23, 0, 0, 0, 87, 0, 0, 5, 20, 0, 12, 0, 5, 0, 12, 0, 120, 0, 0, 5, 5, 0, 12, 0, 62, 4

Offset: 1

Michel Lagneau, Oct 04 2020

nonn

Number of elements in the set {(x, y, z): x|n, y|n, z|n, x < y < z, GCD(x, y, z) > 1}.
Every element of the sequence is repeated indefinitely, for instance:
a(n) = 0 for n = 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 13, ... (Numbers with at most 2 prime factors (counted with multiplicity). See A037143);
a(n) = 1 for n = 8, 27, 125, 343, 1331, 2197, 4913,... (cubes of primes. See A030078);
a(n) = 4 for n = 16, 81, 625, 2401, 14641, 28561, ... (prime(n)^4. See A030514);
a(n) = 5 for n = 12, 18, 20, 28, 44, 45, ... (Numbers which are the product of a prime and the square of a different prime (p^2 * q). See A054753);
a(n) = 12 for n = 30, 42, 66, 70, 78, 102, 105, 110,... (Sphenic numbers: products of 3 distinct primes. See A007304);
a(n) = 20 for n = 64, 729, 15625, 117649, ... (Numbers with 7 divisors. 6th powers of primes. See A030516);
a(n) = 23 for n = 24, 40, 54, 56, 88, 104, 135, 136, ... (Product of the cube of a prime (A030078) and a different prime. See A065036);
a(n) = 36 for n = 36, 100, 196, 225, 441, 484, 676,... (Squares of the squarefree semiprimes (p^2*q^2). See A085986);
a(n) = 62 for n = 48, 80, 112, 162, 176, 208, 272, ... (Product of the 4th power of a prime (A030514) and a different prime (p^4*q). See A178739);
a(n) = 87 for n = 60, 84, 90, 126, 132, 140, 150, 156, ... (Product of exactly four primes, three of which are distinct (p^2*q*r). See A085987);
a(n) = 120 for n = 72, 108, 200, 392, 500, 675, 968, ... (Numbers of the form p^2*q^3, where p,q are distinct primes. See A143610);
It is possible to continue with a(n) = 130, 235, 284, 289, 356, ...

			a(12) = 5 because the divisors of 12 are {1, 2, 3, 4, 6, 12} and GCD(d_i, d_j, d_k) > 1 for the 5 following triples of divisors: (2,4,6), (2,4,12), (2,6,12), (3,6,12) and (4,6,12).

David A. Corneth, Table of n, a(n) for n = 1..10000

Cf. A275387.

with(numtheory):nn:=100:
for n from 1 to nn do:
it:=0:d:=divisors(n):n0:=nops(d):
  for i from 1 to n0-2 do:
   for j from i+1 to n0-1 do:
     for k from j+1 to n0 do:
    if igcd(d[i],d[j],d[k])> 1
       then
       it:=it+1:
       else
      fi:
     od:
     od:
     od:
    printf(`%d, `,it):
   od:

Array[Count[GCD @@ # & /@ Subsets[Divisors[#], {3}], ?(# > 1 &)] &, 81] (* _Michael De Vlieger, Oct 05 2020 *)

a(n) = my(d=divisors(n)); sum(i=1, #d-2, sum (j=i+1, #d-1, sum (k=j+1, #d, gcd([d[i], d[j], d[k]]) > 1))); \\ Michel Marcus, Oct 31 2020

a(n) = {my(f = factor(n), vp = vecprod(f[,1]), d = divisors(vp), res = 0);
for(i = 2, #d, res-=binomial(numdiv(n/d[i]), 3)*(-1)^omega(d[i])); res} \\ David A. Corneth, Nov 01 2020

Name clarified by editors, Oct 31 2020

A382292 Numbers k such that A382290(k) = 1.

Original entry on oeis.org

8, 24, 27, 32, 40, 54, 56, 64, 72, 88, 96, 104, 108, 120, 125, 135, 136, 152, 160, 168, 184, 189, 192, 200, 224, 232, 243, 248, 250, 264, 270, 280, 288, 296, 297, 312, 320, 328, 343, 344, 351, 352, 360, 375, 376, 378, 392, 408, 416, 424, 432, 440, 448, 456, 459, 472, 480, 486, 488, 500

Offset: 1

Amiram Eldar, Mar 21 2025

First differs from A374590 and A375432 at n = 25: A374590(25) = A375432(25) = 216 is not a term of this sequence.
Numbers k such that A382291(k) = 2, i.e., numbers whose number of infinitary divisors is twice the number of their unitary divisors.
Numbers whose prime factorization has a single exponent that is a sum of two distinct powers of 2 (A018900) and all the other exponents, if they exist, are powers of 2. Equivalently, numbers of the form p^e * m, where p is a prime, e is a term in A018900, and m is a term in A138302 that is coprime to p.
If k is a term then k^2 is also a term. If m is a term in A138302 that is coprime to k then k * m is also a term. The primitive terms, i.e., the terms that cannot be generated from smaller terms using these rules, are the numbers of the form p^(2^i+1), where p is prime and i >= 1.
Analogous to A060687, which is the sequence of numbers k with prime excess A046660(k) = 2.
The asymptotic density of this sequence is A271727 * Sum_{p prime} (((1 - 1/p)/f(1/p)) * Sum_{k>=1} 1/p^A018900(k)) = 0.11919967112489084407..., where f(x) = 1 - x^3 + Sum_{k>=2} (x^(2^k)-x^(2^k+1)).

Amiram Eldar, Table of n, a(n) for n = 1..10000

Cf. A018900, A046660, A060687, A271727, A374590, A375432, A382290, A382291.

Subsequences (numbers of the form): A030078 (p^3), A050997 (p^5), A030516 (p^6), A179665 (p^9), A030629 (p^10), A030631 (p^12), A065036 (p^3*q), A178740 (p^5*q), A189987 (p^6*q), A179692 (p^9*q), A143610 (p^2*q^3), A179646 (p^5*q^2), A189990 (p^2*q^6), A179702 (p^4*q^5), A179666 (p^4*q^3), A190464 (p^4*q^6), A163569 (p^3*q^2*r), A189975 (p*q*r^3), A190115 (p^2*q^3*r^4), A381315, A048109.

f[p_, e_] := DigitCount[e, 2, 1] - 1; q[1] = False; q[n_] := Plus @@ f @@@ FactorInteger[n] == 1; Select[Range[500], q]

isok(k) = vecsum(apply(x -> hammingweight(x) - 1, factor(k)[, 2])) == 1;

A119586 Triangle where T(n,m) = (n+1-m)-th positive integer with (m+1) divisors.

Original entry on oeis.org

2, 3, 4, 5, 9, 6, 7, 25, 8, 16, 11, 49, 10, 81, 12, 13, 121, 14, 625, 18, 64, 17, 169, 15, 2401, 20, 729, 24, 19, 289, 21, 14641, 28, 15625, 30, 36, 23, 361, 22, 28561, 32, 117649, 40, 100, 48, 29, 529, 26, 83521, 44, 1771561, 42, 196, 80, 1024, 31, 841, 27

Offset: 1

Leroy Quet, May 31 2006

From Peter Munn, May 17 2023: (Start)
As a square array A(n,m), n, m >= 1, read by ascending antidiagonals, A(n,m) is the n-th positive integer with m+1 divisors.
Thus both formats list the numbers with m+1 divisors in their m-th column. For the corresponding sequences giving numbers with a specific number of divisors see the index entries link.
(End)

			Looking at the 4th row, 7 is the 4th positive integer with 2 divisors, 25 is the 3rd positive integer with 3 divisors, 8 is the 2nd positive integer with 4 divisors and 16 is the first positive integer with 5 divisors. So the 4th row is (7,25,8,16).
The triangle T(n,m) begins:
  n\m:    1     2     3     4     5     6     7
  ---------------------------------------------
   1 :    2
   2 :    3     4
   3 :    5     9     6
   4 :    7    25     8    16
   5 :   11    49    10    81    12
   6 :   13   121    14   625    18    64
   7 :   17   169    15  2401    20   729    24
  ...
Square array A(n,m) begins:
  n\m:     1      2      3       4      5  ...
  --------------------------------------------
   1 :     2      4      6      16     12  ...
   2 :     3      9      8      81     18  ...
   3 :     5     25     10     625     20  ...
   4 :     7     49     14    2401     28  ...
   5 :    11    121     15   14641     32  ...
  ...

Index entries for sequences related to divisors of numbers

Columns: A000040, A001248, A007422, A030514, A030515, A030516, A030626, A030627, A030628, ... (see the index entries link for more).
Cf. A073915.
Diagonals (equivalently, rows of the square array) start: A005179\{1}, A161574.
Cf. A091538.

t[n_, m_] := Block[{c = 0, k = 1}, While[c < n + 1 - m, k++; If[DivisorSigma[0, k] == m + 1, c++ ]]; k]; Table[ t[n, m], {n, 11}, {m, n}] // Flatten (* Robert G. Wilson v, Jun 07 2006 *)

More terms from Robert G. Wilson v, Jun 07 2006

A157294 Decimal expansion of 1575/Pi^6.

Original entry on oeis.org

1, 6, 3, 8, 2, 5, 4, 3, 2, 0, 4, 4, 0, 9, 6, 7, 3, 6, 6, 3, 4, 1, 4, 9, 4, 2, 7, 4, 9, 8, 9, 8, 7, 3, 5, 5, 4, 9, 1, 8, 7, 0, 2, 5, 2, 6, 6, 4, 4, 3, 4, 4, 7, 1, 8, 0, 7, 2, 9, 0, 0, 6, 7, 4, 8, 9, 2, 5, 0, 4, 2, 3, 5, 5, 7, 4, 4, 7, 9, 0, 4, 1, 3, 4, 8, 3, 1, 5, 9, 2, 4, 6, 3, 0, 4, 9, 2, 3, 6, 9, 2, 5, 6, 9, 1

Offset: 1

R. J. Mathar, Feb 26 2009

Equals the asymptotic mean of the abundancy index of the 7-free numbers (numbers that are not divisible by a 7th power other than 1) (Jakimczuk and Lalín, 2022). - Amiram Eldar, May 12 2023

			1.63825432044096736634149427498... = (1+1/2^2+1/2^4+1/2^6)*(1+1/3^2+1/3^4+1/3^6)*(1+1/5^2+1/5^4+1/5^6)*(1+1/7^2+1/7^4+1/7^6)*...

Rafael Jakimczuk and Matilde Lalín, Asymptotics of sums of divisor functions over sequences with restricted factorization structure, Notes on Number Theory and Discrete Mathematics, Vol. 28, No. 4 (2022), pp. 617-634, eq. (1).
Index entries for transcendental numbers.

Cf. A001248, A030514, A030516, A082020, A013661, A013666, A092746, A157290.

Maple
```
evalf(1575/Pi^6) ;
```

RealDigits[1575/Pi^6,10,120][[1]] (* Harvey P. Dale, May 26 2019 *)

1575/Pi^6 \\ Charles R Greathouse IV, Oct 01 2022

Equals Product_{p = primes = A000040} (1+1/p^2+1/p^4+1/p^6).

Equals A013661/A013666 = A082020*A157290 = Product_{i>=1} (1+1/A001248(i)+1/A030514(i)+1/A030516(i)) = 1575*A092746.

A157296 Decimal expansion of 31185/(2*Pi^8).

Original entry on oeis.org

1, 6, 4, 3, 2, 9, 9, 6, 8, 1, 8, 5, 7, 0, 9, 9, 9, 9, 2, 2, 7, 7, 4, 8, 0, 1, 8, 0, 1, 2, 9, 1, 4, 9, 7, 8, 4, 6, 0, 8, 2, 8, 7, 5, 8, 4, 4, 5, 7, 2, 3, 5, 0, 9, 8, 5, 9, 5, 8, 3, 4, 5, 0, 5, 1, 6, 4, 3, 2, 8, 6, 4, 8, 1, 2, 4, 5, 5, 1, 7, 4, 9, 5, 3, 7, 5, 1, 3, 7, 4, 2, 3, 7, 6, 5, 4, 9, 2, 9, 5, 6, 5, 8, 2, 8

Offset: 1

R. J. Mathar, Feb 26 2009

Equals the asymptotic mean of the abundancy index of the 9-free numbers (numbers that are not divisible by a 9th power other than 1) (Jakimczuk and Lalín, 2022). - Amiram Eldar, May 12 2023

			1.64329968185709999227... = (1+1/2^2+1/2^4+1/2^6+1/2^8)*(1+1/3^2+1/3^4+1/3^6+1/3^8)*(1+1/5^2+1/5^4+1/5^6+1/5^8)*...

Rafael Jakimczuk and Matilde Lalín, Asymptotics of sums of divisor functions over sequences with restricted factorization structure, Notes on Number Theory and Discrete Mathematics, Vol. 28, No. 4 (2022), pp. 617-634, eq. (1).
Index entries for transcendental numbers.

Cf. A001248, A030514, A030514, A030516, A082020, A013661, A013668, A092748.

Maple
```
evalf(31185/2/Pi^8) ;
```

RealDigits[31185/(2*Pi^8),10,120][[1]] (* Harvey P. Dale, Mar 30 2018 *)

31185/2/Pi^8 \\ Charles R Greathouse IV, Oct 01 2022

Equals Product_{p = primes = A000040} (1+1/p^2+1/p^4+1/p^6+1/p^8). The variant Product_{p} (1+1/p^2+1/p^6+1/p^8) equals A082020*Product_{p} (1+1/p^6) = A082020*zeta(6)/zeta(12) = 10135125/(691*Pi^8).

Equals A013661/A013668 = Product_{i>=1} (1+1/A001248(i)+1/A030514(i)+1/A030516(i)+1/A030514(i)^2) = 15592.5*A092748.

cp's OEIS Frontend

A280346 Numbers with 79 divisors.

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A280347 Numbers with 83 divisors.

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A280349 Numbers with 89 divisors.

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A122220 a(n) = (prime(n)^6 - prime(n)^2)/20.

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A137489 Numbers with 26 divisors.

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A336530 Number of triples of divisors d_i < d_j < d_k of n such that gcd(d_i, d_j, d_k) > 1.

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A382292 Numbers k such that A382290(k) = 1.

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