login
A306488
Number of ways of expressing n as a + b + c, with a, b, and c positive integers, gcd(a, b) = 1, but gcd(a, c) and gcd(b, c) both greater than 1.
1
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 4, 0, 4, 0, 1, 0, 9, 0, 7, 1, 4, 1, 15, 0, 13, 1, 4, 2, 16, 0, 24, 4, 10, 1, 29, 0, 32, 4, 5, 3, 41, 0, 38, 2, 17, 6, 54, 1, 43, 6, 26, 10, 70, 0, 65, 9, 20, 11, 68, 1, 86, 14, 35, 2, 99, 1, 99, 15, 18, 16, 104, 1, 125, 10, 53, 19, 134, 0, 114, 21, 58
OFFSET
0,18
REFERENCES
F. Barrera, B. Recamán and S. Wagon, Problem 12044, Amer. Math. Monthly 125 (2018), p. 466.
LINKS
EXAMPLE
a(11) = 1 because of the ten partitions of 11 into three parts, only 6 + 3 + 2 satisfies the conditions. But a(210) = 0, because 210 does not have any partition that satisfies the conditions.
MATHEMATICA
a[n_] := Length@ Select[ IntegerPartitions[ n, {3}], (t = Sort[GCD @@@ Subsets[#, {2}]]; t[[1]] == 1 && t[[2]] > 1 && t[[3]] > 1) &]; a /@ Range[0, 87] (* Giovanni Resta, Feb 20 2019 *)
PROG
(Sage)
def a(n):
if n < 3: return 0
r = 0
t = [False, True, True]
for p in Partitions(n, length=3, min_part=2, max_slope=-1):
s = sorted(gcd(a, b) > 1 for a, b in Subsets(p, 2))
r += int(s == t)
return r
[a(n) for n in (0..100)]
CROSSREFS
Sequence in context: A028618 A147986 A147988 * A230031 A019920 A246130
KEYWORD
nonn
AUTHOR
Freddy Barrera, Feb 18 2019
STATUS
approved