|
|
A068050
|
|
Number of values of k, 1<=k<=n, for which floor(n/k) is prime.
|
|
4
|
|
|
0, 1, 1, 1, 2, 2, 3, 2, 2, 4, 5, 3, 4, 5, 6, 5, 6, 5, 6, 6, 7, 9, 10, 6, 7, 9, 9, 9, 10, 10, 11, 9, 10, 12, 14, 11, 12, 13, 14, 13, 14, 13, 14, 14, 15, 17, 18, 13, 14, 16, 17, 18, 19, 17, 19, 18, 19, 21, 22, 18, 19, 20, 21, 19, 21, 22, 23, 23, 24, 26, 27, 21, 22, 23, 24, 24, 26, 27
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,5
|
|
LINKS
|
|
|
FORMULA
|
|
|
EXAMPLE
|
a(10) = 4 as floor(10/k) for k = 1 to 10 is 10,5,3,2,2,1,1,1,1,1, respectively; this is prime for k = 2,3,4,5.
|
|
MATHEMATICA
|
a[n_] := Length[Select[Table[Floor[n/i], {i, 1, n}], PrimeQ]]
Table[Count[Table[Floor[n/k], {k, n}], _?PrimeQ], {n, 80}] (* Harvey P. Dale, Nov 19 2022 *)
|
|
PROG
|
(Haskell)
a068050 n = length [k | k <- [1..n], a010051 (n `div` k) == 1]
|
|
CROSSREFS
|
|
|
KEYWORD
|
easy,nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|