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A038109 Divisible exactly by the square of a prime. 12
4, 9, 12, 18, 20, 25, 28, 36, 44, 45, 49, 50, 52, 60, 63, 68, 72, 75, 76, 84, 90, 92, 98, 99, 100, 108, 116, 117, 121, 124, 126, 132, 140, 144, 147, 148, 150, 153, 156, 164, 169, 171, 172, 175, 180, 188, 196, 198, 200, 204, 207, 212, 220, 225, 228, 234, 236, 242 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers for which at least one prime factor exponent is exactly 2.

Sometimes called squarefull numbers, although that term is usually reserved for A001694. - N. J. A. Sloane, Jul 22 2012

The asymptotic density of this sequence is 1 - A330596 = 0.2514647... - Amiram Eldar, Aug 12 2020

LINKS

R. J. Mathar, Table of n, a(n) for n = 1..1247

EXAMPLE

20=5*2*2 is divisible by 2^2.

MAPLE

isA038109 := proc(n)

    local p;

    for p in ifactors(n)[2] do

        if op(2, p) = 2 then

            return true;

        end if;

    end do:

    false ;

end proc: # R. J. Mathar, Dec 08 2015

# second Maple program:

q:= n-> ormap(i-> i[2]=2, ifactors(n)[2]):

select(q, [$1..300])[];  # Alois P. Heinz, Aug 12 2020

MATHEMATICA

Select[Range[250], MemberQ[Transpose[FactorInteger[#]][[2]], 2]&] (* Harvey P. Dale, Sep 24 2012 *)

PROG

(PARI) is(n)=#select(n->n==2, Set(factor(n)[, 2])) \\ Charles R Greathouse IV, Sep 17 2015

CROSSREFS

Cf. A001694, A013929, A284017, A284018.

Sequence in context: A081619 A336594 A304365 * A067259 A060687 A286228

Adjacent sequences:  A038106 A038107 A038108 * A038110 A038111 A038112

KEYWORD

nonn,easy

AUTHOR

Felice Russo

EXTENSIONS

Corrected and extended by Erich Friedman

STATUS

approved

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Last modified November 27 23:47 EST 2020. Contains 338685 sequences. (Running on oeis4.)