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A181833
The number of positive integers <= n that are not strongly prime to n.
8
0, 0, 2, 3, 4, 4, 6, 5, 6, 7, 9, 5, 10, 7, 10, 11, 12, 6, 14, 7, 14, 15, 16, 5, 18, 13, 17, 13, 20, 7, 24, 9, 18, 19, 22, 15, 28, 10, 22, 19, 28, 9, 32, 9, 26, 27, 30, 5, 34, 17, 33, 25, 32, 7, 38, 23, 36, 29, 34, 5, 46
OFFSET
0,3
COMMENTS
k is strongly prime to n iff k is relatively prime to n and k does not divide n-1.
a(n) = n - phi(n) + tau(n-1) if n > 0 and a(0) = 0.
Here phi(n) = A000010(n) and tau(n) = A000005(n).
EXAMPLE
a(11) = 11 - card({3,4,6,7,8,9}) = 5.
MAPLE
with(numtheory):
A181833 := n -> `if`(n=0, 0, n-phi(n)+tau(n-1));
A181833a := n -> n - A181830(n);
MATHEMATICA
a[n_] := Select[Range[n], Not[CoprimeQ[#, n] && !Divisible[n-1, #]] &] // Length; a[1] = 0; Table[a[n], {n, 0, 60}] (* Jean-François Alcover, Jun 28 2013 *)
KEYWORD
nonn
AUTHOR
Peter Luschny, Nov 17 2010
STATUS
approved