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A336432
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Number of ordered quadruples of divisors (d_i, d_j, d_k, d_m) of n such that GCD(d_i, d_j, d_k, d_m) > 1.
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1
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 16, 0, 0, 0, 1, 0, 3, 0, 5, 0, 0, 0, 29, 0, 0, 0, 16, 0, 3, 0, 1, 1, 0, 0, 74, 0, 1, 0, 1, 0, 16, 0, 16, 0, 0, 0, 98, 0, 0, 1, 15, 0, 3, 0, 1, 0, 3, 0, 181, 0, 0, 1, 1, 0, 3, 0, 74, 1, 0, 0, 98, 0, 0, 0, 16, 0, 98, 0, 1, 0, 0, 0, 220, 0, 1, 1, 29, 0, 3, 0
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OFFSET
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1,24
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COMMENTS
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Number of elements in the set {(x, y, z, w): x|n, y|n, z|n, w|n , x < y < z < w, GCD(x, y, z, w) > 1}.
Every element of the sequence is repeated indefinitely, for instance:
a(n) = 0 for n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 25, 26, ... (Numbers k such that product of proper divisors of k is <= k; i.e., product of divisors of k is <= k^2). See A007964);
a(n) = 1 for n = 12, 16, 18, 20, 28, 44, 45, 50, 52, 63, 68, 75, 76, 81, 92, 98, 99,... (Either 4th power of a prime, or product of a prime and the square of a different prime. See A080258);
a(n) = 5 for n = 32, 243, 3125, 16807, ... (Fifth powers of primes.. See A050997);
a(n) = 15 for n = 64, 729, 15625, 117649, ... (Numbers with 7 divisors. 6th powers of primes. See A030516).
a(n) = 16 for n = 24, 40, 56, 135, 189, 297, 351, 459, ... (numbers of the form p^3*q, p and q primes with q > p).
a(n) = 17 for n = 54, 88, 104, 136, 152, 184, 232, 248, ... (numbers of the form p^i*q^j, p and q primes and (i, j) = (3, 1) or (1, 3).
a(n) = 30 for n = 36, 225, 441, 1225, 3025, 4225, 5929, ... (numbers of the form p^2*q^2, p and q primes.
It is possible to continue with a(n) = 74, 75, 78, 107, 110, 112, 114, ...
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LINKS
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EXAMPLE
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a(30) = 3 because the divisors of 30 are {1, 2, 3, 5, 6, 10, 15, 30} and GCD(d_i, d_j, d_k, d_m) > 1 for the 4 following quadruples of divisors: (2,6,10,30), (3,6,15,30) and (5,10,15,30).
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MAPLE
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with(numtheory):nn:=100:
for n from 1 to nn do:
it:=0:d:=divisors(n):n0:=nops(d):
for i from 1 to n0-3 do:
for j from i+1 to n0-2 do:
for k from j+1 to n0-1 do:
for l from k+1 to n0 do:
if igcd(d[i], d[j], d[k], d[l])> 1
then
it:=it+1:
else
fi:
od:
od:
od:
od:
printf(`%d, `, it):
od:
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MATHEMATICA
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PROG
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(PARI) a(n) = my(d=divisors(n)); sum(i=1, #d-3, sum (j=i+1, #d-2, sum (k=j+1, #d-1, sum (m=k+1, #d, gcd([d[i], d[j], d[k], d[m]]) > 1)))); \\ Michel Marcus, Oct 31 2020
(PARI) 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]), 4)*(-1)^omega(d[i])); res} \\ David A. Corneth, Oct 31 2020
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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