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A276710
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Composite numbers m such that Product_{k=0..m} binomial(m,k) is divisible by m^(m-1).
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2
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36, 40, 63, 80, 84, 90, 105, 108, 132, 144, 150, 154, 160, 165, 168, 175, 176, 180, 182, 195, 198, 200, 208, 210, 220, 260, 264, 270, 273, 275, 280, 286, 288, 297, 300, 306, 308, 312, 315, 320, 324, 330, 340, 351, 357, 360, 364, 374, 378, 380, 385, 390
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OFFSET
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1,1
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COMMENTS
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The numbers Product_{k=0..m} binomial(m,k) form the sequence A001142(m). When m is a prime, the m-1 factors for 0 < k < m are all divisible by m and therefore A001142(m) is divisible by m^(m-1). When m is a composite, this is generally not so, except for the numbers listed here (a variety of pseudoprimes).
Conjecture, tested so far up to m = 3828: "When m belongs to this list, Product_{k=0..m} binomial(m,k) is divisible also by m^m". Since this is impossible for prime m (see, e.g., A109874), the conjecture is equivalent to the statement "m is prime if and only if Product_{k=0..m} binomial(m,k) is divisible by m^(m-1) but not by m^m".
This conjecture has been proved, see corollary 3 in PDF link below.
The set of all numbers in this sequence has natural density 1 - log(2), see theorem 2 in PDF link below. (End)
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LINKS
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EXAMPLE
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Since 36 is composite and 36^35 divides Product_{k=1..36} binomial(36,k), 36 is a member. In addition, it turns out that 36^36 also divides the product.
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MATHEMATICA
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Select[DeleteCases[Range[2, 400], p_ /; PrimeQ@ p], Divisible[Product[Binomial[#, k], {k, 0, #}], #^(# - 1)] &] (* Michael De Vlieger, Sep 16 2016 *)
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PROG
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(PARI) generator() = { /* Operates on two pre-defined integer vectors a, b of the same size. a(n) receives the terms of this sequence, while b(n) receives 0 if n^n|Product(binomial(n, k)), or 1 otherwise, and serves exclusively to test the conjecture. */
my (m=1, n=0, p); for(k=1, #a, a[k]=0; b[k]=0);
while(1, m++; p=prod(k=1, m-1, binomial(m, k));
if((p%m^(m-1)==0)&&(!isprime(m)), n++; a[n]=m;
if(p%m^m==0, b[n]=0, b[n]=1); if(n==#a, break)));
}
a=vector(1000); b=a; generator();
a /* Displays the result.
Note: execution was interrupted due to excessive execution time */
(Python)
from itertools import islice
from math import comb
from sympy import nextprime
def A276710_gen(): # generator of terms
p, q = 3, 5
while True:
for m in range(p+1, q):
r = m**(m-1)
c = 1
for k in range(m+1):
c = c*comb(m, k) % r
if c == 0:
yield m
p, q = q, nextprime(q)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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