cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-4 of 4 results.

A383694 Primitive exponential unitary abundant numbers: the powerful terms of A383693.

Original entry on oeis.org

900, 1764, 4356, 4500, 4900, 6084, 10404, 12348, 12996, 19044, 22500, 30276, 34596, 44100, 47916, 49284, 60516, 66564, 79092, 79524, 86436, 88200, 101124, 108900, 112500, 125316, 132300, 133956, 152100, 161604, 176400, 176868, 181476, 191844, 213444, 217800, 220500
Offset: 1

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Author

Amiram Eldar, May 05 2025

Keywords

Comments

First differs from its subsequence A383698 at n = 11.
For squarefree numbers k, eusigma(k) = k, where eusigma is the sum of exponential unitary divisors function (A322857). Thus, if m is a term (eusigma(m) > 2*m) and k is a squarefree number coprime to m, then eusigma(k*m) = eusigma(k) * eusigma(m) = k * eusigma(m) > 2*k*m, so k*m is an exponential unitary abundant number. Therefore, the sequence of exponential unitary abundant numbers (A383693) can be generated from this sequence by multiplying with coprime squarefree numbers.
The least odd term is a(1455) = 225450225, and the least term that is coprime to 6 is 1117347505588495206025.

Examples

			900 is a term since eusigma(900) = 2160 > 2 * 900, and 900 = 2^2 * 3^2 * 5^2 is a powerful number.
6300 is exponential unitary abundant, since eusigma(6300) = 15120 > 2 * 6300, but it is not a powerful number: 6300 = 2^2 * 3^2 * 5^2 * 7. Thus it is not in this sequence. It can be generated as a term of A383693 from a(1) = 900 by 7 * 900 = 6300, since 7 is squarefree and gcd(7, 900) = 1.

Links

Crossrefs

Intersection of A001694 and A383693.
A383698 is a subsequence.
Cf. A005117, A322857, A328136.

Programs

  • Mathematica
    fun[p_, e_] := DivisorSum[e, p^# &, GCD[#, e/#] == 1 &]; q[n_] := Min[(f = FactorInteger[n])[[;; , 2]]] > 1 && Times @@ fun @@@ f > 2*n; Select[Range[250000], q]
  • PARI
    fun(p, e) = sumdiv(e, d, if(gcd(d, e/d) == 1, p^d));
isok(k) = {my(f = factor(k)); ispowerful(f) && prod(i = 1, #f~, fun(f[i, 1], f[i, 2])) > 2*k;}

A323332 The Dedekind psi function values of the powerful numbers, A001615(A001694(n)).

Original entry on oeis.org

1, 6, 12, 12, 24, 30, 36, 48, 72, 56, 96, 144, 108, 180, 216, 132, 150, 192, 288, 182, 336, 360, 432, 360, 324, 384, 576, 306, 648, 392, 380, 672, 720, 864, 672, 792, 900, 768, 552, 1152, 750, 1296, 1080, 1092, 972, 1344, 1440, 870, 1728, 2160, 992, 1584
Offset: 1

Views

Author

Amiram Eldar, Jan 11 2019

Keywords

Comments

The sum of the reciprocals of all the terms of this sequence is Pi^2/6 (A013661).
The asymptotic density of a sequence S that possesses the property that an integer k is a term if and only if its powerful part, A057521(k) is a term, is (1/zeta(2)) * Sum_{n>=1, A001694(n) is a term of S} 1/a(n). Examples for such sequences are the e-perfect numbers (A054979), the exponential abundant numbers (A129575), and other sequences listed in the Crossrefs section. - Amiram Eldar, May 06 2025

Links

Crossrefs

Cf. A001615, A001694, A013661, A082695, A112526.
Sequences whose density can be calculated using this sequence: A054979, A129575, A307958, A308053, A321147, A322858, A323310, A328135, A339936, A340109, A364990, A382061, A383693, A383695, A383697.

Programs

  • Mathematica
    psi[1]=1; psi[n_] := n * Times@@(1+1/Transpose[FactorInteger[n]][[1]]); psi /@ Join[{1}, Select[Range@ 1200, Min@ FactorInteger[#][[All, 2]] > 1 &]] (* after T. D. Noe at A001615 and Harvey P. Dale at A001694 *)
  • Python
    from math import isqrt, prod
from sympy import mobius, integer_nthroot, primefactors
def A323332(n):
    def squarefreepi(n): return int(sum(mobius(k)*(n//k**2) for k in range(1, isqrt(n)+1)))
    def bisection(f,kmin=0,kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def f(x):
        c, l = n+x-squarefreepi(integer_nthroot(x,3)[0]), 0
        j = isqrt(x)
        while j>1:
            k2 = integer_nthroot(x//j**2,3)[0]+1
            w = squarefreepi(k2-1)
            c -= j*(w-l)
            l, j = w, isqrt(x//k2**3)
        return c+l
    a = primefactors(m:=bisection(f,n,n))
    return m*prod(p+1 for p in a)//prod(a) # Chai Wah Wu, Sep 14 2024

A383697 Exponential squarefree exponential abundant numbers: numbers k such that A361174(k) > 2*k.

Original entry on oeis.org

900, 1764, 4356, 4500, 4900, 6084, 6300, 8820, 9900, 10404, 11700, 12348, 12996, 14700, 15300, 17100, 19044, 19404, 20700, 21780, 22932, 26100, 27900, 29988, 30276, 30420, 30492, 31500, 33300, 33516, 34596, 36900, 38700, 40572, 42300, 42588, 44100, 47700, 47916, 49284, 49500
Offset: 1

Views

Author

Amiram Eldar, May 06 2025

Keywords

Comments

Subsequence of A383693 and first differs from it at n = 21.
All the terms are nonsquarefree numbers (A013929), since A361174(k) = k if k is a squarefree number (A005117).
The least odd term is a(198045) = 225450225, and the least term that is coprime to 6 is a(9.815...*10^17) = 1117347505588495206025.
The least term that is not an exponentially squarefree number (A209061) is a(8.85...*10^1324) = 2^4 * Product_{k=2..248} prime(k)^2 = 1.00786...*10^1328.
The asymptotic density of this sequence is Sum_{n>=1} f(A383698(n)) = 0.000878475..., where f(n) = (6/(Pi^2*n))*Product_{prime p|n}(p/(p+1)).

Links

Crossrefs

Subsequence of A013929, A129575 and A383693.
A383698 is a subsequence.
Cf. A005117, A209061, A361174.

Programs

  • Mathematica
    f[p_, e_] := DivisorSum[e, p^# &, SquareFreeQ[#] &]; q[k_] := Times @@ f @@@ FactorInteger[k] > 2*k; Select[Range[1000], q]
  • PARI
    ff(p, e) = sumdiv(e, d, if(issquarefree(d), p^d, 0));
isok(k) = {my(f = factor(k)); prod(i=1, #f~, ff(f[i, 1], f[i, 2])) > 2*k; }

A383695 Exponential infinitary abundant numbers that are not exponential unitary abundant: numbers k such that A361175(k) > 2*k >= A322857(k).

Original entry on oeis.org

476985600, 815673600, 1018886400, 1177862400, 1493049600, 2014214400, 2373638400, 2712326400, 3756614400, 3863865600, 4744454400, 5218617600, 5246841600, 6234681600, 7928121600, 8108755200, 8245036800, 8972409600, 9062726400, 9824774400, 10502150400, 10603756800
Offset: 1

Views

Author

Amiram Eldar, May 05 2025

Keywords

Comments

Exponential infinitary abundant numbers are numbers k such that A361175(k) > 2*k.
All the exponential unitary abundant numbers (A383693) are also exponential infinitary abundant numbers. There are numbers that are exponential infinitary abundant and not exponential unitary abundant. The least is: a(1) = 476985600, which is the 427970th exponential infinitary abundant number.
All the terms are nonsquarefree numbers (A013929), since A361175(k) = k if k is a squarefree number (A005117).
The asymptotic density of this sequence is Sum_{n>=1} f(A383696(n)) = 1.9875...*10^(-9), where f(n) = (6/(Pi^2*n))*Product_{prime p|n}(p/(p+1)). The relative density of this sequence within the exponential infinitary abundant numbers is 2.215... * 10^(-6).

Links

Crossrefs

Cf. A005117, A322857, A361175.
Subsequence of A013929 and A129575.
A383696 is a subsequence.

Programs

  • Mathematica
    seq[max_] := Module[{prim = seqA383696[max], s = {}, sq}, Do[sq = Select[Range[Floor[max/p]], CoprimeQ[p, #] && SquareFreeQ[#] &]; s = Join[s, p*sq], {p, prim}]; Union[s]]; seq[10^10] (* using the function seqA383696 from A383696 *)
  • PARI
    list(lim) = {my(p = listA383696(lim), s = []); for(i = 1, #p, s = concat(s, apply(x -> p[i]*x, select(x -> gcd(x, p[i]) == 1 && issquarefree(x), vector(lim\p[i], j, j))))); Set(s);} \\ using the function listA383696 from A383696
Showing 1-4 of 4 results.

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