A179983 Positive integers n such that, if k appears in n's prime signature, k-1 appears at least as often as k (for any integer k > 1).

1, 2, 3, 5, 6, 7, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 26, 28, 29, 30, 31, 33, 34, 35, 37, 38, 39, 41, 42, 43, 44, 45, 46, 47, 50, 51, 52, 53, 55, 57, 58, 59, 60, 61, 62, 63, 65, 66, 67, 68, 69, 70, 71, 73, 74, 75, 76, 77, 78, 79, 82, 83, 84, 85, 86, 87, 89, 90

Offset

1,2

Comments

Numbers n such that A181819(n) is a member of A025487.

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Example

The prime signature of 20 = 2^2*5 is (2,1). Since the largest number appearing in 20's prime signature is 2, and 1 appears as many times as 2, 20 is a member of this sequence.

Maple

isA179983 := proc(n)
    local es, me, k ;
    # list of exponents in prime signature
    es := [seq(op(2, pe), pe =ifactors(n)[2])] ;
    # maximum exponent
    me := max(op(es)) ;
    for k from me to 2 by -1 do
        if numboccur(es, k-1) < numboccur(es, k) then
            return false;
        end if;
    end do:
    true ;
end proc:
for n from 1 to 100 do
    if isA179983(n) then
        printf("%d, ", n) ;
    end if;
end do: # R. J. Mathar, Mar 21 2023

Crossrefs

Includes all squarefree numbers (A005117); also includes all members of A054753, A085987, A163569, A182862, A182863.

Cf. A025487, A181819.

Sequence in context: A339741 A317090 A361393 * A065872 A028741 A119316

Adjacent sequences: A179980 A179981 A179982 * A179984 A179985 A179986

Keyword

nonn

Author

Matthew Vandermast, Jan 15 2011

Status

approved