A061798 Number of sums i^3 + j^3 that occur more than once for 1<=i<=j<=n.

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 7, 7, 8, 8, 8, 9, 10, 10, 10, 10, 10, 10, 10, 10, 12, 12, 12, 13, 13, 14, 15, 16, 16, 16, 17, 17, 19, 19, 19, 19, 20, 20, 20, 21, 23, 24, 24, 24, 25, 25, 25, 25

Offset: 1

Author

Labos Elemer, Jun 22 2001

Keywords

• nonn

Links

• Seiichi Manyama, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000217, A011541, A061791.

Programs

PARI

a(n) = my(v=vector(2*n^3, i, 0)); for(i=1, n, for(j=i, n, v[i^3+j^3]+=1)); sum(i=1, #v, v[i]>1); \\ Seiichi Manyama, May 14 2024

Ruby

def A(n)
  h = {}
  (1..n).each{|i|
    (i..n).each{|j|
      k = i * i * i + j * j * j
      if h.has_key?(k)
        h[k] += 1
      else
        h[k] = 1
      end
    }
  }
  h.to_a.select{|i| i[1] > 1}.size
end
def A061798(n)
  (1..n).map{|i| A(i)}
end
p A061798(80) # Seiichi Manyama, May 14 2024