A078174 -id:A078174 - cp's OEIS frontend

A070005 Arithmetic mean of prime factors of n is an integer and n is neither a prime nor power of a prime.

15, 21, 33, 35, 39, 42, 45, 51, 55, 57, 63, 65, 69, 75, 77, 78, 84, 85, 87, 91, 93, 95, 99, 105, 110, 111, 114, 115, 117, 119, 123, 126, 129, 133, 135, 141, 143, 145, 147, 153, 155, 156, 159, 161, 168, 170, 171, 175, 177, 183, 185, 186, 187, 189, 195, 201, 203

Offset: 1

Labos Elemer, Apr 11 2002

			n=33=3*11, mean=(3+11)/2=6.

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Cf. A070006, A001414, A008472.

Cf. A000961, A010055; subsequence of A078174.

a070005 n = a070005_list !! (n-1)
a070005_list = filter ((== 0) . a010055) a078174_list
-- Reinhard Zumkeller, Jun 01 2013

ffi[x_] := Flatten[FactorInteger[x]] lf[x_] := Length[FactorInteger[x]] ba[x_] := Table[Part[ffi[x], 2*w-1], {w, 1, lf[x]}] ep[x_] := Table[Part[ffi[x], 2*w], {w, 1, lf[x]}] Do[s=Apply[Plus, ba[n]]/lf[n]; If[IntegerQ[s]&&Greater[lf[n], 1], Print[n]], {n, 2, 1000}]

lista(nn) = {for (n=2, nn, f = factor(n); if ((#f~ != 1) && (sum(k=1, #f~, f[k,1]) % #f~ == 0), print1(n, ", ")););} \\ Michel Marcus, Mar 28 2015

A176587 Numbers such that arithmetic mean of distinct prime factors is not an integer.

Original entry on oeis.org

1, 6, 10, 12, 14, 18, 20, 22, 24, 26, 28, 30, 34, 36, 38, 40, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 66, 68, 70, 72, 74, 76, 80, 82, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108, 112, 116, 118, 120, 122, 124, 130, 132, 134, 136, 138, 140, 142, 144

Offset: 1

Jaroslav Krizek, Apr 21 2010

nonn

Complement of A078174.

			For a(14) = 36: 36 = 2^2*3^3; (2+3)/2 is not integer.

Join[{1},Select[Range[300],!IntegerQ[Mean[FactorInteger[#][[All,1]]]]&]] (* Harvey P. Dale, Aug 05 2022 *)

Corrected and extended by Harvey P. Dale, Aug 05 2022

A357710 Number of integer compositions of n with integer geometric mean.

Original entry on oeis.org

0, 1, 2, 2, 3, 4, 4, 8, 4, 15, 17, 22, 48, 40, 130, 88, 287, 323, 543, 1084, 1145, 2938, 3141, 6928, 9770, 15585, 29249, 37540, 78464, 103289, 194265, 299752, 475086, 846933, 1216749, 2261920, 3320935, 5795349, 9292376, 14825858, 25570823, 39030115, 68265801, 106030947, 178696496

Offset: 0

Gus Wiseman, Oct 15 2022

nonn

			The a(6) = 4 through a(9) = 15 compositions:
  (6)       (7)        (8)         (9)
  (33)      (124)      (44)        (333)
  (222)     (142)      (2222)      (1224)
  (111111)  (214)      (11111111)  (1242)
            (241)                  (1422)
            (412)                  (2124)
            (421)                  (2142)
            (1111111)              (2214)
                                   (2241)
                                   (2412)
                                   (2421)
                                   (4122)
                                   (4212)
                                   (4221)
                                   (111111111)

The unordered version (partitions) is A067539, ranked by A326623.
Compositions with integer average are A271654, partitions A067538.
Subsets whose geometric mean is an integer are A326027.
The version for factorizations is A326028.
The strict case is A339452, partitions A326625.
These compositions are ranked by A357490.
A011782 counts compositions.
Cf. A025047, A051293, A078174, A078175, A102627, A320322, A326622, A326624, A326641, A357182, A357183.

Table[Length[Select[Join @@ Permutations/@IntegerPartitions[n],IntegerQ[GeometricMean[#]]&]],{n,0,15}]

from math import prod, factorial
from sympy import integer_nthroot
from sympy.utilities.iterables import partitions
def A357710(n): return sum(factorial(s)//prod(factorial(d) for d in p.values()) for s,p in partitions(n,size=True) if integer_nthroot(prod(a**b for a, b in p.items()),s)[1]) if n else 0 # Chai Wah Wu, Sep 24 2023

More terms from David A. Corneth, Oct 17 2022

A070007 Arithmetic mean of distinct primes dividing n is a square number.

Original entry on oeis.org

15, 42, 45, 65, 75, 77, 84, 87, 126, 135, 141, 168, 225, 247, 252, 258, 261, 285, 294, 301, 325, 335, 336, 357, 375, 378, 405, 410, 423, 429, 481, 504, 516, 539, 588, 589, 591, 618, 671, 672, 675, 717, 756, 767, 774, 783, 785, 820, 845, 847, 855, 882, 986

Offset: 1

Labos Elemer, Apr 11 2002

nonn

Subset of A078174. [R. J. Mathar, Sep 20 2008]

			n=1972=2*17*29: mean=(2+17+29)/3=48/3=16, a square.

Michael De Vlieger, Table of n, a(n) for n = 1..10000

Cf. A070005, A070006, A001414, A008472.

Select[Range[2, 1000], IntegerQ@ Sqrt[Total[First /@ FactorInteger@ #]/PrimeNu@ #] &] (* Michael De Vlieger, Mar 28 2015 *)

lista(nn) = {for (n=2, nn, f = factor(n); if ((#f~ != 1) && (type(q=sum(k=1, #f~, f[k,1])/#f~) == "t_INT") && issquare(q), print1(n, ", ")););} \\ Michel Marcus, Mar 28 2015

A174894 Numbers such that the arithmetic mean of their distinct prime factors and the arithmetic mean of all of their prime factors are both integers.

Original entry on oeis.org

2, 3, 4, 5, 7, 8, 9, 11, 13, 15, 16, 17, 19, 21, 23, 25, 27, 29, 31, 32, 33, 35, 37, 39, 41, 42, 43, 47, 49, 51, 53, 55, 57, 59, 61, 64, 65, 67, 69, 71, 73, 77, 78, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 101

Offset: 1

Jaroslav Krizek, Apr 01 2010

nonn

Subsequence of A078174 and A078175.

Complement of A176552. [From Jaroslav Krizek, Apr 21 2010]

			For a(11) = 16: 16 = 2^4; both (2+2+2+2)/4 and 2/1 are integers.

Harvey P. Dale, Table of n, a(n) for n = 1..1000

mdmaQ[n_]:=With[{fi=FactorInteger[n]},AllTrue[{Mean[Flatten[Table[#[[1]],#[[2]]]&/@fi]],Mean[fi[[;;,1]]]},IntegerQ]]; Select[Range[ 2,110],mdmaQ] (* Harvey P. Dale, Nov 16 2024 *)

Definition clarified by Harvey P. Dale, Nov 16 2024

A363895 Floor of the average of the distinct prime factors of n.

Original entry on oeis.org

2, 3, 2, 5, 2, 7, 2, 3, 3, 11, 2, 13, 4, 4, 2, 17, 2, 19, 3, 5, 6, 23, 2, 5, 7, 3, 4, 29, 3, 31, 2, 7, 9, 6, 2, 37, 10, 8, 3, 41, 4, 43, 6, 4, 12, 47, 2, 7, 3, 10, 7, 53, 2, 8, 4, 11, 15, 59, 3, 61, 16, 5, 2, 9, 5, 67, 9, 13, 4, 71, 2, 73, 19, 4, 10, 9, 6, 79

Offset: 2

Darío Clavijo, Jun 26 2023

Cf. A126594, A008472, A001221, A078174, A323171, A323172.

a[n_] := Floor[Mean[FactorInteger[n][[;; , 1]]]]; Array[a, 100, 2] (* Amiram Eldar, Jun 27 2023 *)

a(n) = my(p = factor(n)[, 1]); vecsum(p)\#p; \\ Amiram Eldar, Jun 29 2023

from sympy import factorint
def a(n):
  P = factorint(n).keys()
  return int(sum(P)/len(P))
print([a(n) for n in range(2, 85)])

a(p^n) = p, p prime, n >= 1.
a(n) = floor(A008472(n)/A001221(n)).
a(n) = floor(A323171(n)/A323172(n)).

A285510 Numbers k such that the average of the squarefree divisors of k is an integer.

Original entry on oeis.org

1, 3, 5, 6, 7, 9, 11, 12, 13, 14, 15, 17, 18, 19, 21, 22, 23, 24, 25, 27, 28, 29, 30, 31, 33, 35, 36, 37, 38, 39, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 53, 54, 55, 56, 57, 59, 60, 61, 62, 63, 65, 66, 67, 69, 70, 71, 72, 73, 75, 76, 77, 78, 79, 81, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 101

Offset: 1

Ilya Gutkovskiy, Apr 20 2017

nonn

Numbers n such that A034444(n)|A048250(n).
Numbers n such that 2^omega(n)|psi(rad(n)), where omega() is the number of distinct prime divisors (A001221), psi() is the Dedekind psi function (A001615) and rad() is the squarefree kernel (A007947).
From Robert Israel, Apr 24 2017: (Start)
All odd numbers are in the sequence.
A positive even number is in the sequence if and only if at least one of its prime factors is in A002145.
Thus this is the complement of 2*A072437 in the positive numbers.
(End)

			44 is in the sequence because 44 has 6 divisors {1, 2, 4, 11, 22, 44} among which 4 are squarefree {1, 2, 11, 22} and (1 + 2 + 11 + 22)/4 = 9 is an integer.

Robert Israel, Table of n, a(n) for n = 1..10000
Index entries for sequences related to sums of divisors.

Cf. A001221, A001615, A002145, A003601, A005117, A007947, A023886, A034444, A048250, A072437, A078174, A103826, A206778.

filter:= n -> n::odd or has(numtheory:-factorset(n) mod 4, 3):
select(filter, [$1..1000]); # Robert Israel, Apr 24 2017

Select[Range[100], IntegerQ[Total[Select[Divisors[#], SquareFreeQ]] / 2^PrimeNu[#]] &]
Select[Range[110],IntegerQ[Mean[Select[Divisors[#],SquareFreeQ]]]&] (* Harvey P. Dale, Apr 11 2018 *)
Select[Range[100], IntegerQ[Times @@ ((1 + FactorInteger[#][[;; , 1]])/2)] &] (* Amiram Eldar, Jul 01 2022 *)

a(n) ~ n (conjecture).

Conjecture is true, since A072437 has density 0. - Robert Israel, Apr 24 2017

A339452 Number of compositions (ordered partitions) of n into distinct parts such that the geometric mean of the parts is an integer.

Original entry on oeis.org

1, 1, 1, 1, 3, 1, 7, 1, 1, 5, 1, 1, 9, 7, 3, 1, 3, 1, 7, 11, 13, 1, 7, 1, 11, 35, 25, 31, 27, 5, 157, 1, 31, 131, 39, 31, 33, 37, 183, 179, 135, 157, 7, 265, 3, 871, 187, 865, 259, 879, 867, 179, 1593, 6073, 1593, 271, 5995, 149, 6661, 2411, 1509, 997, 1045, 5887

Offset: 1

Ilya Gutkovskiy, Dec 05 2020

nonn

			a(10) = 5 because we have [10], [9, 1], [1, 9], [8, 2] and [2, 8].

Eric W. Weisstein's World of Mathematics, Geometric Mean
Index entries for sequences related to compositions

For partitions we have A326625, non-strict A067539 (ranked by A326623).
The version for subsets is A326027.
For arithmetic mean we have A339175, non-strict A271654.
The non-strict case is counted by A357710, ranked by A357490.
A032020 counts strict compositions.
A067538 counts partitions with integer average.
A078175 lists numbers whose prime factors have integer average.
A320322 counts partitions whose product is a perfect power.
Cf. A051293, A078174, A096199, A102627, A326622, A326624, A326028, A326641, A326645, A335405.

Table[Length[Select[Join@@Permutations/@IntegerPartitions[n],UnsameQ@@#&&IntegerQ[GeometricMean[#]]&]],{n,0,15}] (* Gus Wiseman, Oct 30 2022 *)

A303482 Numbers k such that the average of all distinct prime factors of all positive integers <= k is an integer.

Original entry on oeis.org

2, 5, 81, 10742, 10130527, 1041972864, 23292549600

Offset: 1

Ilya Gutkovskiy, Apr 24 2018

Numbers k such that A013939(k)|A024924(k).

			5 is in the sequence because the distinct prime factors of 2, 3, 4, and 5 are 2, 3, 2 and 5 respectively and their average (2 + 3 + 2 + 5) / 4 = 3 is an integer. - _David A. Corneth_, Apr 26 2018

Eric Weisstein's World of Mathematics, Distinct Prime Factors

Cf. A001221, A008472, A013939, A024924, A078174, A078175, A226647, A284755, A303480.

s = t = 0; k = 2; lst = {}; While[k < 1000000000, p = #[[1]] & /@ FactorInteger@ k; s = s + Plus @@ p; t = t + Length@ p; If[ Mod[s, t] == 0, AppendTo[lst, k]]; k++]; lst (* Robert G. Wilson v, Apr 26 2018 *)

a(5) from Daniel Suteu, Apr 24 2018

a(6)-a(7) from Giovanni Resta, Apr 26 2018

A357490 Numbers k such that the k-th composition in standard order has integer geometric mean.

Original entry on oeis.org

1, 2, 3, 4, 7, 8, 10, 15, 16, 17, 24, 31, 32, 36, 42, 63, 64, 69, 70, 81, 88, 98, 104, 127, 128, 136, 170, 255, 256, 277, 278, 282, 292, 325, 326, 337, 344, 354, 360, 394, 418, 424, 511, 512, 513, 514, 515, 528, 547, 561, 568, 640, 682, 768, 769, 785, 792, 896

Offset: 1

Gus Wiseman, Oct 16 2022

nonn

The k-th composition in standard order (graded reverse-lexicographic, A066099) is obtained by taking the set of positions of 1's in the reversed binary expansion of k, prepending 0, taking first differences, and reversing again. This gives a bijective correspondence between nonnegative integers and integer compositions.

			The terms together with their corresponding compositions begin:
   1: (1)
   2: (2)
   3: (1,1)
   4: (3)
   7: (1,1,1)
   8: (4)
  10: (2,2)
  15: (1,1,1,1)
  16: (5)
  17: (4,1)
  24: (1,4)
  31: (1,1,1,1,1)
  32: (6)
  36: (3,3)
  42: (2,2,2)
  63: (1,1,1,1,1,1)
  64: (7)
  69: (4,2,1)

For regular mean we have A096199, counted by A271654 (partitions A067538).
Subsets whose geometric mean is an integer are counted by A326027.
The unordered version (partitions) is A326623, counted by A067539.
The strict case is counted by A339452, partitions A326625.
These compositions are counted by A357710.
A078175 lists numbers whose prime factors have integer average.
A320322 counts partitions whose product is a perfect power.
Cf. A051293, A078174, A102627, A301987, A326622, A326624, A326028, A326567/A326568, A326641, A326645, A335405, A357184.

stc[n_]:=Differences[Prepend[Join@@Position[Reverse[IntegerDigits[n,2]],1],0]]//Reverse;
Select[Range[0,1000],IntegerQ[GeometricMean[stc[#]]]&]

cp's OEIS Frontend

A070005 Arithmetic mean of prime factors of n is an integer and n is neither a prime nor power of a prime.

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A176587 Numbers such that arithmetic mean of distinct prime factors is not an integer.

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A357710 Number of integer compositions of n with integer geometric mean.

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A070007 Arithmetic mean of distinct primes dividing n is a square number.

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A174894 Numbers such that the arithmetic mean of their distinct prime factors and the arithmetic mean of all of their prime factors are both integers.

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A363895 Floor of the average of the distinct prime factors of n.

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A285510 Numbers k such that the average of the squarefree divisors of k is an integer.

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A339452 Number of compositions (ordered partitions) of n into distinct parts such that the geometric mean of the parts is an integer.

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