OFFSET
1,1
COMMENTS
Or numbers n>=9 having a divisor t^2 > 1, where t=k/m, 1<= m < k, such that n == n/t^2 (mod 5).
Or positive numbers n such that if n == 0 (mod 5), then it divisible by 5^3 or by the square of some other prime; otherwise n divisible by k^2, such that there is a k_1, 0< k_1 <k with the condition k_1^2 == k^2 (mod 5).
A generalization see in our seqfan list from Jun 13 (correction Jun 14) 2016.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
FORMULA
Let A(x) be the number of a(n)<=x. Then A(x)/x ~ 1 - 149/(20*Pi^2) = 1 - 7.45/Pi^2 = 0.24515718... as x goes to infinity. - Vladimir Shevelev, Jun 15 2016; corrected by Charles R Greathouse IV, Jun 15 2016
EXAMPLE
9 is member, since 4 == 9 (mod 5) and 4*9 is a square;
32 is member, since 2 == 32 (mod 5) and 2*32 is a square;
45 is member, since 5 == 45 (mod 5) and 5*45 is a square.
MATHEMATICA
Select[Range@ 250, Function[n, Count[Sqrt[# n] & /@ Select[Range[n - 1], Mod[#, 5] == Mod[n, 5] &], k_ /; IntegerQ@ k] > 0]] (* Michael De Vlieger, Jun 14 2016 *)
PROG
(PARI) isok(n)=j = n-5; while (j >0, if (issquare(j*n), return (1)); j -= 5; ); 0; \\ Michel Marcus, Jun 14 2016
(PARI) is(n)=!issquarefree(n/if(n%5, if(n%4, 1, 4), 5)) \\ Charles R Greathouse IV, Jun 15 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vladimir Shevelev, Jun 12 2016
EXTENSIONS
Corrected and extended by Michel Marcus, Jun 14 2016
STATUS
approved