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A348719 Numbers k delivering the records in the number of partitions of divisors of k into two sets of size > 1 with an integer arithmetic mean in each set. 0
1, 6, 12, 18, 20, 24, 30, 42, 48, 60, 120, 180, 240, 360, 420, 480, 504, 540, 630, 720, 840, 1080, 1260, 1440, 1680, 2160, 2520, 5040, 7560, 10080, 15120, 20160 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
The corresponding record values are 0, 1, 3, 4, 6, 16, 20, 21, 29, 198, 1542, 3448, 9055, 86081, 245107, 245145, 245336, 249208, 250595, 4844170, 31669733, ...
LINKS
EXAMPLE
6 is the smallest number whose set of divisors can be partitioned into two disjoint sets whose arithmetic means are both integers: {1, 3} and {2, 6}.
12 is the smallest number whose set of divisors can be partitioned into two disjoint sets whose arithmetic means are both integers in three ways: ({1, 3}, {2, 4, 6, 12}), ({2, 6}, {1, 3, 4, 12}) and ({4, 12}, {1, 2, 3, 6}).
MATHEMATICA
q[d_] := Length[d] > 1 && IntegerQ @ Mean[d]; c[n_] := Count[Subsets[(d = Divisors[n])], _?(q[#] && q[Complement[d, #]] &)]/2; cm = -1; s = {}; Do[If[(c1 = c[n]) > cm, cm = c1; AppendTo[s, n]], {n, 1, 240}]; s
PROG
(Python)
from itertools import count, islice, combinations
from sympy import divisors
def A348719_gen(): # generator of terms
c = 0
yield 1
for n in count(2):
divs = tuple(divisors(n, generator=True))
l, b = len(divs), sum(divs)
if l>=4 and 2**(l-1)-l>c:
m = sum(1 for k in range(2, (l-1>>1)+1) for p in combinations(divs, k) if not ((s:=sum(p))%k or (b-s)%(l-k)))
if l&1 == 0:
k = l>>1
m += sum(1 for p in combinations(divs, k) if 1 in p and not ((s:=sum(p))%k or (b-s)%k))
if m > c:
yield n
c = m
A348719_list = list(islice(A348719_gen(), 10)) # Chai Wah Wu, Sep 24 2023
CROSSREFS
Sequence in context: A088723 A228870 A291022 * A316221 A138939 A221220
KEYWORD
nonn,more
AUTHOR
Amiram Eldar, Oct 31 2021
EXTENSIONS
a(22)-a(23) from Chai Wah Wu, Sep 24 2023
a(24) from Chai Wah Wu, Sep 26 2023
a(25)-a(32) added and definition corrected by Max Alekseyev, Feb 01 2024
STATUS
approved

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Last modified February 25 04:28 EST 2024. Contains 370309 sequences. (Running on oeis4.)