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A239929 Numbers n with the property that the symmetric representation of sigma(n) has two parts. 20
3, 5, 7, 10, 11, 13, 14, 17, 19, 22, 23, 26, 29, 31, 34, 37, 38, 41, 43, 44, 46, 47, 52, 53, 58, 59, 61, 62, 67, 68, 71, 73, 74, 76, 78, 79, 82, 83, 86, 89, 92, 94, 97, 101, 102, 103, 106, 107, 109, 113, 114, 116, 118, 122, 124, 127, 131, 134, 136, 137, 138 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All odd primes are in the sequence because the parts of the symmetric representation of sigma(prime(i)) are [m, m], where m = (1 + prime(i))/2, for i >= 2.

There are no odd composite numbers in this sequence.

First differs from A173708 at a(13).

Since sigma(p*q) >= 1 + p + q + p*q for odd p and q, the symmetric representation of sigma(p*q) has more parts than the two extremal ones of size (p*q + 1)/2; therefore, the above comments are true. - Hartmut F. W. Hoft, Jul 16 2014

From Hartmut F. W. Hoft, Sep 16 2015: (Start)

The following two statements are equivalent:

  (1) The symmetric representation of sigma(n) has two parts, and

  (2) n = q * p where q is in A174973, p is prime, and 2 * q < p.

For a proof see the link and also the link in A071561.

This characterization allows for much faster computation of numbers in the sequence - function a239929F[] in the Mathematica section - than computations based on Dyck paths. The function a239929Stalk[] gives rise to the associated irregular triangle whose columns are indexed by A174973 and whose rows are indexed by A065091, the odd primes. (End)

From Hartmut F. W. Hoft, Dec 06 2016: (Start)

For the respective columns of the irregular triangle with fixed m: k = 2^m * p, m >= 1, 2^(m+1) < p and p prime:

(a) each number k is representable as the sum of 2^(m+1) but no fewer consecutive positive integers [since 2^(m+1) < p].

(b) each number k has 2^m as largest divisor <= sqrt(k) [since 2^m < sqrt(k) < p].

(c) each number k is of the form 2^m * p with p prime [by definition].

m = 1: (a) A100484 even semiprimes (except 4 and 6)

       (b) A161344 (except 4, 6 and 8)

       (c) A001747 (except 2, 4 and 6)

m = 2: (a) A270298

       (b) A161424 (except 16, 20, 24, 28 and 32)

       (c) A001749 (except 8, 12, 20 and 28)

m = 3: (a) A270301

       (b) A162528 (except 64, 72, 80, 88, 96, 104, 112 and 128)

       (c) sequence not in OEIS

b(i,j) = A174973(j) * {1,5) mod 6 * A174973(j), for all i,j >= 1; see A091999 for j=2. (End)

LINKS

Table of n, a(n) for n=1..61.

Hartmut F. W. Hoft, Proof of Characterization Theorem

FORMULA

Entries b(i, j) in the irregular triangle with rows indexed by i>=1 and columns indexed by j>=1 (alternate indexing of the example):

b(i,j) = A000040(i+1) * A174973(j) where A000040(i+1) > 2 * A174973(j). - Hartmut F. W. Hoft, Dec 06 2016

EXAMPLE

From Hartmut F. W. Hoft, Sep 16 2015: (Start)

a(23) = 52 = 2^2 * 13 = q * p with q = 4 in A174973 and 8 < 13 = p.

a(59) = 136 = 2^3 * 17 = q * p with q = 8 in A174973 and 16 < 17 = p.

The first six columns of the irregular triangle through prime 37:

   1    2    4    6    8   12 ...

  -------------------------------

   3

   5   10

   7   14

  11   22   44

  13   26   52   78

  17   34   68  102  136

  19   38   76  114  152

  23   46   92  138  184

  29   58  116  174  232  348

  31   62  124  186  248  372

  37   74  148  222  296  444

  ...

(End)

MAPLE

isA174973 := proc(n)

    option remember;

    local k, dvs;

    dvs := sort(convert(numtheory[divisors](n), list)) ;

    for k from 2 to nops(dvs) do

        if op(k, dvs) > 2*op(k-1, dvs) then

            return false;

        end if;

    end do:

    true ;

end proc:

A174973 := proc(n)

    if n = 1 then

        1;

    else

        for a from procname(n-1)+1 do

            if isA174973(a) then

                return a;

            end if;

        end do:

    end if;

end proc:

isA239929 := proc(n)

    local i, p, j, a73;

    for i from 1 do

        p := ithprime(i+1) ;

        if p > n then

            return false;

        end if;

        for j from 1 do

            a73 := A174973(j) ;

            if a73 > n then

                break;

            end if;

            if p > 2*a73 and n = p*a73 then

                return true;

            end if;

        end do:

    end do:

end proc:

for n from 1 to 200 do

    if isA239929(n) then

        printf("%d, ", n) ;

    end if;

end do: # R. J. Mathar, Oct 04 2018

MATHEMATICA

(* sequence of numbers k for m <= k <= n having exactly two parts *)

(* Function a237270[] is defined in A237270 *)

a239929[m_, n_]:=Select[Range[m, n], Length[a237270[#]]==2&]

a239929[1, 260] (* data *)

(* Hartmut F. W. Hoft, Jul 07 2014 *)

(* test for membership in A174973 *)

a174973Q[n_]:=Module[{d=Divisors[n]}, Select[Rest[d] - 2 Most[d], #>0&]=={}]

a174973[n_]:=Select[Range[n], a174973Q]

(* compute numbers satisfying the condition *)

a239929Stalk[start_, bound_]:=Module[{p=NextPrime[2 start], list={}}, While[start p<=bound, AppendTo[list, start p]; p=NextPrime[p]]; list]

a239929F[n_]:=Sort[Flatten[Map[a239929Stalk[#, n]&, a174973[n]]]]

a239929F[138] (* data *)(* Hartmut F. W. Hoft, Sep 16 2015 *)

CROSSREFS

Column 2 of A240062.

Cf. A000203, A006254, A065091, A071561, A174973, A196020, A236104, A235791, A237591, A237593, A237270, A237271, A238443, A239660, A239663, A239665, A239931-A239934, A244050, A245062, A262626.

Cf. A000040, A001747, A001749, A091999, A100484, A161344, A161424, A162528, A270298, A270301. - Hartmut F. W. Hoft, Dec 06 2016

Sequence in context: A241561 A241008 A173708 * A246955 A167907 A306639

Adjacent sequences:  A239926 A239927 A239928 * A239930 A239931 A239932

KEYWORD

nonn

AUTHOR

Omar E. Pol, Apr 06 2014

EXTENSIONS

Extended beyond a(56) by Michel Marcus, Apr 07 2014

STATUS

approved

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Last modified March 30 09:49 EDT 2020. Contains 333125 sequences. (Running on oeis4.)