OFFSET
1,2
FORMULA
a(n) = A006218(n)+n-1. [Enrique Pérez Herrero, Sep 25 2009]
EXAMPLE
The 4x4 Redheffer matrix:
1,1,1,1
1,1,0,0
1,0,1,0
1,1,0,1
contains 11 nonzero elements.
MATHEMATICA
A161886[n_] := Plus @@ Table[DivisorSigma[0, i], {i, 1, n}] + n - 1 (* Enrique Pérez Herrero, Sep 25 2009 *)
A161886[n_] := Total[Table[ Boole[Divisible[i, j] || (i == 1)], {i, 1, n}, {j, 1, n}], Infinity] (* Enrique Pérez Herrero, Sep 25 2009 *)
A161889[n_] := Plus @@ Plus @@ Table[Boole[Divisible[i, j] || (i == 1)], {i, 1, n}, {j, 1, n}] (* Enrique Pérez Herrero, Sep 28 2009 *)
A161889[n_] := Sum[Ceiling[n/i], {i, 1, n}] + DivisorSigma[0, n] - 1 (* Enrique Pérez Herrero, Sep 28 2009 *)
PROG
(Python)
from math import isqrt
def A161886(n): return (lambda m: 2*sum(n//k for k in range(1, m+1))+n-1-m*m)(isqrt(n)) # Chai Wah Wu, Oct 09 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Mats Granvik, Jun 21 2009
EXTENSIONS
Edited by N. J. A. Sloane, Jun 26 2009
STATUS
approved