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 1-10 of 640 results. Next

A061652 Even superperfect numbers: 2^(p-1) where 2^p-1 is a Mersenne prime (A000668).

Original entry on oeis.org

2, 4, 16, 64, 4096, 65536, 262144, 1073741824, 1152921504606846976, 309485009821345068724781056, 81129638414606681695789005144064, 85070591730234615865843651857942052864
Offset: 1

Views

Author

Jason Earls, Jun 16 2001

Keywords

Comments

It is conjectured that there are no odd superperfect numbers, in which case this coincides with A019279.
The number of divisors of a(n) is equal to A000043(n). - Omar E. Pol, Feb 29 2008
The sum of divisors of a(n) is equal to A000668(n), the n-th Mersenne prime. - Omar E. Pol, Mar 11 2008
Largest proper divisor of A072868(n). - Omar E. Pol, Apr 25 2008
Indices of hexagonal numbers (A000384) that are also even perfect numbers. [Omar E. Pol, Aug 26 2008]
Except for the first perfect number 6, this sequence is the greatest common divisor of a perfect number (A000396) and its arithmetic derivative (A003415). - Giorgio Balzarotti, Apr 21 2011
If n is in the sequence then n is a solution to the equation phi(sigma(x)) = 2x-2. It seems that there is no other solution to this equation. - Jahangeer Kholdi, Sep 09 2014
The sum of sums of elements of subsets of divisors of a(n), i.e. A229335(a(n)), is a perfect number (A000396). - Jaroslav Krizek, Nov 02 2017

References

  • James J. Tattersall, Elementary Number Theory in Nine Chapters, Cambridge University Press, 1999, page 147.

Crossrefs

Programs

  • Mathematica
    2^(Select[Range[512], PrimeQ[2^# - 1] &] - 1) (* Alonso del Arte, Apr 22 2011 *)
    2^(MersennePrimeExponent[Range[15]]-1) (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 20 2021 *)
  • PARI
    forprime(p=2,1e3,if(ispseudoprime(2^p-1),print1(2^(p-1)", "))) \\ Charles R Greathouse IV, Mar 14 2012

Formula

a(n) = 2^(A090748(n)). - Lekraj Beedassy, Dec 07 2007
a(n) = (1 + A000668(n))/2. - Omar E. Pol, Mar 11 2008
a(n) = 2^A000043(n)/2 = A072868(n)/2 = A032742(A072868(n)). - Omar E. Pol, Apr 25 2008

A046528 Numbers that are a product of distinct Mersenne primes (elements of A000668).

Original entry on oeis.org

1, 3, 7, 21, 31, 93, 127, 217, 381, 651, 889, 2667, 3937, 8191, 11811, 24573, 27559, 57337, 82677, 131071, 172011, 253921, 393213, 524287, 761763, 917497, 1040257, 1572861, 1777447, 2752491, 3120771, 3670009, 4063201, 5332341, 7281799, 11010027, 12189603
Offset: 1

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Author

Keywords

Comments

Or, numbers n such that the sum of the divisors of n is a power of 2, see A094502.
Or, numbers n such that the number of divisors of n and the sum of the divisors of n are both powers of 2.
n is a product of distinct Mersenne primes iff sigma(n) is a power of 2: see exercise in Sivaramakrishnan, or Shallit.
Sequence gives n > 1 such that sigma(n) = 2*phi(sigma(n)). - Benoit Cloitre, Feb 22 2002
The graph of this sequence shows a discontinuity at the 4097th number because there is a large relative gap between the 12th and 13th Mersenne primes, A000043. Other discontinuities can be predicted using A078426. - T. D. Noe, Oct 12 2006
Supersequence of A051281 (numbers n such that sigma(n) is a power of tau(n)). Conjecture: numbers n such that sigma(n) = tau(n)^(a/b), where a, b are integers >= 1. Example: sigma(93) = 128 = tau(93)^(7/2) = 4^(7/2). - Jaroslav Krizek, May 04 2013

Examples

			a(20) = 82677 = 3*7*31*127, whose sum of divisors is 131072 = 2^17;
a(27) = 1040257 = 127*8191, whose sum of divisors is 1048576 = 2^20.
		

References

  • J.-M. De Koninck & A. Mercier, 1001 Problèmes en Théorie Classique des Nombres, Problem 264 pp. 188, Ellipses Paris 2004.
  • R. Sivaramakrishnan, Classical Theory of Arithmetic Functions. Dekker, 1989.

Crossrefs

Cf. A000668, A000043, A056652 (product of Mersenne primes), A306204.

Programs

  • Maple
    mersennes:= [seq(numtheory:-mersenne([i]),i=1..10)]:
    sort(select(`<`,map(convert,combinat:-powerset(mersennes),`*`),numtheory:-mersenne([11]))); # Robert Israel, May 01 2016
  • Mathematica
    {1}~Join~TakeWhile[Times @@@ Rest@ Subsets@ # // Sort, Function[k, k <= Last@ #]] &@ Select[2^Range[0, 31] - 1, PrimeQ] (* Michael De Vlieger, May 01 2016 *)
  • PARI
    isok(n) = (n==1) || (ispower(sigma(n), , &r) && (r==2)); \\ Michel Marcus, Dec 10 2013

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A000668} (1 + 1/p) = 1.5855588879... (A306204) - Amiram Eldar, Jan 06 2021

Extensions

More terms from Benoit Cloitre, Feb 22 2002
Further terms from Jon Hart, Sep 22 2006
Entry revised by N. J. A. Sloane, Mar 20 2007
Three more terms from Michel Marcus, Dec 10 2013

A139256 Twice even perfect numbers. Also a(n) = M(n)*(M(n)+1), where M(n) is the n-th Mersenne prime A000668(n).

Original entry on oeis.org

12, 56, 992, 16256, 67100672, 17179738112, 274877382656, 4611686016279904256, 5316911983139663489309385231907684352, 383123885216472214589586756168607276261994643096338432
Offset: 1

Views

Author

Omar E. Pol, Apr 22 2008

Keywords

Comments

Also, twice perfect numbers, if there are no odd perfect numbers.
If there are no odd perfect numbers, essentially the same as A065125. - R. J. Mathar, May 23 2008
The sum of reciprocals of even divisors of a(n) equals 1. Proof: Let n = (2^m - 1)*2^m where 2^m - 1 is a Mersenne prime. The sum of reciprocals of even divisors of n is s1 + s2 where: s1 = 1/2 + 1/4 + ... + 1/2^m = (2^m - 1)/2^m and s2 = s1/(2^m - 1) => s1 + s2 = 1. - Michel Lagneau, Jul 17 2013

Examples

			a(3) = 992 because the third Mersenne prime A000668(3) is 31 and 31*(31+1) = 31*32 = 992.
a(3) = 992 because the sum of the divisors of the third perfect number is 1 + 2 + 4 + 8 + 16 + 31 + 62 + 124 + 248 + 496 = 992. - _Omar E. Pol_, Dec 05 2016
From _Omar E. Pol_, Aug 13 2021: (Start)
Illustration of initial terms in which a(n) is represented as the sum of the divisors of the n-th even perfect number P(n).
-------------------------------------------------------------------------
  n  P(n) a(n)  Diagram:   1                                           2
-------------------------------------------------------------------------
                           _                                           _
                          | |                                         | |
                          | |                                         | |
                       _ _| |                                         | |
                      |    _|                                         | |
                 _ _ _|  _|                                           | |
  1    6   12   |_ _ _ _|                                             | |
                                                                      | |
                                                                      | |
                                                                      | |
                                                                      | |
                                                                      | |
                                                                      | |
                                                                      | |
                                                             _ _ _ _ _| |
                                                            |  _ _ _ _ _|
                                                            | |
                                                         _ _| |
                                                     _ _|  _ _|
                                                    |    _|
                                                   _|  _|
                                                  |  _|
                                             _ _ _| |
                                            |  _ _ _|
                                            | |
                                            | |
                                            | |
                 _ _ _ _ _ _ _ _ _ _ _ _ _ _| |
  2   28   56   |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _|
.
a(n) equals the area (also the number of cells) in the n-th diagram.
For n = 3, P(3) = 496 and a(3) = 992, the diagram is too large to include here. To draw that diagram note that the lengths of the line segments of the smallest Dyck path are [248, 83, 42, 25, 17, 13, 9, 7, 6, 5, 5, 3, 4, 2, 3, 2, 2, 2, 2, 2, 1, 2, 1, 2, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 2, 2, 2, 2, 3, 2, 4, 3, 5, 5, 6, 7, 9, 13, 17, 25, 42, 83, 248] and the lengths of the line segments of the largest Dyck path are [249, 83, 42, 25, 17, 13, 9, 7, 6, 5, 5, 3, 4, 2, 3, 2, 2, 2, 2, 2, 1, 2, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 2, 2, 2, 2, 3, 2, 4, 3, 5, 5, 6, 7, 9, 13, 17, 25, 42, 83, 249]. For a definition of these numbers related to partitions into consecutive parts see A237591. (End)
		

Crossrefs

Programs

Formula

a(n) = A000668(n)*(A000668(n)+1).
a(n) = 2*A000396(n), if there are no odd perfect numbers.
a(n) = A000203(A000396(n)) = A001065(A000396(n)) + A000396(n), assuming there are no odd perfect numbers. - Omar E. Pol, Dec 04 2016

Extensions

More terms from Omar E. Pol, Jun 07 2012

A124477 Numbers k such that 24k+7 is a Mersenne prime (A000668).

Original entry on oeis.org

0, 1, 5, 341, 5461, 21845, 89478485, 96076792050570581, 25790417485112089060398421, 6760803201217223474649083762005, 7089215977519551322153637654828504405
Offset: 1

Views

Author

Artur Jasinski, Dec 17 2006

Keywords

Comments

Note that 2^m - 1 can be expressed as 24*k+7 whenever m is an odd integer >= 3. - Robert Israel, Jul 08 2014

Crossrefs

Programs

  • Maple
    seq((numtheory:-mersenne([i+1])-7)/24, i=1..20); # Robert Israel, Jul 08 2014
  • PARI
    for(n=0, 1e20, k=0; if(ispseudoprime(24*n+7), while(2^k-1 < 24*n+7, k++); if(24*n+7==2^k-1, print1(n, ", ")))) \\ Felix Fröhlich, Jul 04 2014
    
  • PARI
    lista(nn) = {vmps = readvec("b000043.txt"); if (nn== 0, nn = #vmps); for (i=1, nn, mpi = 2^vmps[i]-8; if ((mpi % 24) == 0, print1(mpi/24, ", ")););} \\ Michel Marcus, Jul 05 2014

Formula

a(n) = (2^A000043(n+1)-8)/24. - Jeppe Stig Nielsen, Sep 17 2020

Extensions

a(11) corrected by Michel Marcus, Jul 05 2014

A335431 Numbers of the form q*(2^k), where q is one of the Mersenne primes (A000668) and k >= 0.

Original entry on oeis.org

3, 6, 7, 12, 14, 24, 28, 31, 48, 56, 62, 96, 112, 124, 127, 192, 224, 248, 254, 384, 448, 496, 508, 768, 896, 992, 1016, 1536, 1792, 1984, 2032, 3072, 3584, 3968, 4064, 6144, 7168, 7936, 8128, 8191, 12288, 14336, 15872, 16256, 16382, 24576, 28672, 31744, 32512, 32764, 49152, 57344, 63488, 65024, 65528, 98304, 114688, 126976, 130048, 131056, 131071
Offset: 1

Views

Author

Antti Karttunen, Jun 28 2020

Keywords

Comments

Numbers of the form 2^k * ((2^p)-1), where p is one of the primes in A000043, and k >= 0.
Numbers k such that A000265(k) is in A000668.
Numbers k for which A331410(k) = 1.
Numbers k that themselves are not powers of two, but for which A335876(k) = k+A052126(k) is [a power of 2].
Conjecture: This sequence gives all fixed points of map n -> A332214(n) and its inverse n -> A332215(n). See also notes in A029747 and in A163511.

Crossrefs

Cf. A000043, A000396 (even terms form a subsequence), A000668 (primes present), A335882, A341622.
Row 1 of A335430.
Positions of 1's in A331410, in A364260, and in A364251 (characteristic function).
Subsequence of A054784.

Programs

  • Mathematica
    qs = 2^MersennePrimeExponent[Range[6]] - 1; max = qs[[-1]]; Reap[Do[n = 2^k*q; If[n <= max, Sow[n]], {k, 0, Log2[max]}, {q, qs}]][[2, 1]] // Union (* Amiram Eldar, Feb 18 2021 *)
  • PARI
    A000265(n) = (n>>valuation(n,2));
    isA000668(n) = (isprime(n)&&!bitand(n,1+n));
    isA335431(n) = isA000668(A000265(n));

Formula

A332214(a(n)) = A332215(a(n)) = a(n) for all n.
Sum_{n>=1} 1/a(n) = 2 * A173898 = 1.0329083578... - Amiram Eldar, Feb 18 2021

A139294 a(n) = 2^(2p - 1), where p is the n-th Mersenne prime A000668(n).

Original entry on oeis.org

32, 8192, 2305843009213693952, 14474011154664524427946373126085988481658748083205070504932198000989141204992
Offset: 1

Views

Author

Omar E. Pol, Apr 13 2008

Keywords

Comments

Next terms have 4932, 78913, 315652, 1292913986, and 1388255822130839283 decimal digits. - Jens Kruse Andersen, Jul 14 2014

Crossrefs

Programs

  • Mathematica
    A000668 := Select[2^Range[500] - 1, PrimeQ]; Table[2^(2*A000668[[n]] - 1), {n, 1, 5}] (* G. C. Greubel, Oct 03 2017 *)
    a[n_] := 2^(2^(MersennePrimeExponent[n] + 1) - 3); Array[a, 4] (* Amiram Eldar, Jul 10 2025 *)
  • PARI
    \p 100
    print1("a(n): "); forprime(q=2, 7, p=2^q-1; if(isprime(p), print1(2^(2*p-1)", ")));
    print1("\nNumber of digits in a(n): "); forprime(q=2, 127, p=2^q-1; if(isprime(p), print1(ceil((2*p-1)*log(2)/log(10))", "))) \\ Jens Kruse Andersen, Jul 14 2014

Formula

a(n) = 2^(2*A000668(n)-1).

A248932 Decimal expansion of 2^2203 - 1, the 16th Mersenne prime A000668(16).

Original entry on oeis.org

1, 4, 7, 5, 9, 7, 9, 9, 1, 5, 2, 1, 4, 1, 8, 0, 2, 3, 5, 0, 8, 4, 8, 9, 8, 6, 2, 2, 7, 3, 7, 3, 8, 1, 7, 3, 6, 3, 1, 2, 0, 6, 6, 1, 4, 5, 3, 3, 3, 1, 6, 9, 7, 7, 5, 1, 4, 7, 7, 7, 1, 2, 1, 6, 4, 7, 8, 5, 7, 0, 2, 9, 7, 8, 7, 8, 0, 7, 8, 9, 4, 9, 3, 7, 7, 4, 0, 7, 3, 3, 7, 0, 4, 9, 3, 8, 9, 2, 8, 9, 3, 8, 2, 7, 4
Offset: 664

Views

Author

Arkadiusz Wesolowski, Oct 17 2014

Keywords

Comments

The 13th through the 17th Mersenne primes were found in 1952 by Raphael M. Robinson, using SWAC.

Examples

			14759799152141802350848986227373817363120661453331697751477712164785702...
		

Crossrefs

Cf. A169684 = A000668(11), A169681 = A000668(12), A169685 = A000668(13), A204063 = A000668(14), A248931 = A000668(15), A248933 = A000668(17), A248934 = A000668(18), A248935 = A000668(19), A248936 = A000668(20).

Programs

  • Magma
    Reverse(Intseq(2^2203-1));
    
  • Mathematica
    RealDigits[2^2203-1,10,120][[1]] (* Harvey P. Dale, Oct 09 2017 *)
  • PARI
    eval(Vec(Str(2^2203-1)))

Formula

2^A000043(16) - 1.

A204063 Decimal expansion of 2^607 - 1, the 14th Mersenne prime A000668(14).

Original entry on oeis.org

5, 3, 1, 1, 3, 7, 9, 9, 2, 8, 1, 6, 7, 6, 7, 0, 9, 8, 6, 8, 9, 5, 8, 8, 2, 0, 6, 5, 5, 2, 4, 6, 8, 6, 2, 7, 3, 2, 9, 5, 9, 3, 1, 1, 7, 7, 2, 7, 0, 3, 1, 9, 2, 3, 1, 9, 9, 4, 4, 4, 1, 3, 8, 2, 0, 0, 4, 0, 3, 5, 5, 9, 8, 6, 0, 8, 5, 2, 2, 4, 2, 7, 3, 9, 1, 6, 2, 5, 0, 2, 2, 6, 5, 2, 2, 9, 2, 8, 5, 6, 6, 8, 8, 8, 9
Offset: 183

Views

Author

M. F. Hasler, Jan 09 2013

Keywords

Examples

			2^607-1 = 531 * 10^180 +
137992816767098689588206552468627329593117727031923199444138 * 10^120 +
200403559860852242739162502265229285668889329486246501015346 * 10^60 +
579337652707239409519978766587351943831270835393219031728127.
		

Crossrefs

Cf. A169684 = A000668(11), A169681 = A000668(12), A169685 = A000668(13), A248931 = A000668(15), A248932 = A000668(16), A248933 = A000668(17), A248934 = A000668(18), A248935 = A000668(19), A248936 = A000668(20).

Programs

Formula

2^A000043(14)-1.

A248931 Decimal expansion of 2^1279 - 1, the 15th Mersenne prime A000668(15).

Original entry on oeis.org

1, 0, 4, 0, 7, 9, 3, 2, 1, 9, 4, 6, 6, 4, 3, 9, 9, 0, 8, 1, 9, 2, 5, 2, 4, 0, 3, 2, 7, 3, 6, 4, 0, 8, 5, 5, 3, 8, 6, 1, 5, 2, 6, 2, 2, 4, 7, 2, 6, 6, 7, 0, 4, 8, 0, 5, 3, 1, 9, 1, 1, 2, 3, 5, 0, 4, 0, 3, 6, 0, 8, 0, 5, 9, 6, 7, 3, 3, 6, 0, 2, 9, 8, 0, 1, 2, 2, 3, 9, 4, 4, 1, 7, 3, 2, 3, 2, 4, 1, 8, 4, 8, 4, 2, 4
Offset: 386

Views

Author

Arkadiusz Wesolowski, Oct 17 2014

Keywords

Comments

The 13th through the 17th Mersenne primes were found in 1952 by Raphael M. Robinson, using SWAC.

Examples

			10407932194664399081925240327364085538615262247266704805319112350403608...
		

Crossrefs

Cf. A169684 = A000668(11), A169681 = A000668(12), A169685 = A000668(13), A204063 = A000668(14), A248932 = A000668(16), A248933 = A000668(17), A248934 = A000668(18), A248935 = A000668(19), A248936 = A000668(20).

Programs

  • Magma
    Reverse(Intseq(2^1279-1));
    
  • Mathematica
    RealDigits[2^1279 - 1, 10, 100][[1]] (* G. C. Greubel, Oct 03 2017 *)
  • PARI
    eval(Vec(Str(2^1279-1)))

Formula

Equals 2^A000043(15) - 1.

A248933 Decimal expansion of 2^2281 - 1, the 17th Mersenne prime A000668(17).

Original entry on oeis.org

4, 4, 6, 0, 8, 7, 5, 5, 7, 1, 8, 3, 7, 5, 8, 4, 2, 9, 5, 7, 1, 1, 5, 1, 7, 0, 6, 4, 0, 2, 1, 0, 1, 8, 0, 9, 8, 8, 6, 2, 0, 8, 6, 3, 2, 4, 1, 2, 8, 5, 9, 9, 0, 1, 1, 1, 1, 9, 9, 1, 2, 1, 9, 9, 6, 3, 4, 0, 4, 6, 8, 5, 7, 9, 2, 8, 2, 0, 4, 7, 3, 3, 6, 9, 1, 1, 2, 5, 4, 5, 2, 6, 9, 0, 0, 3, 9, 8, 9, 0, 2, 6, 1, 5, 3
Offset: 687

Views

Author

Arkadiusz Wesolowski, Oct 17 2014

Keywords

Comments

The 13th through the 17th Mersenne primes were found in 1952 by Raphael M. Robinson, using SWAC.
The digits of this prime were published on page 167 of Nordisk Mathematisk Tidskrift 2 (1954).

Examples

			44608755718375842957115170640210180988620863241285990111199121996340468...
		

Crossrefs

Cf. A169684 = A000668(11), A169681 = A000668(12), A169685 = A000668(13), A204063 = A000668(14), A248931 = A000668(15), A248932 = A000668(16), A248934 = A000668(18), A248935 = A000668(19), A248936 = A000668(20).

Programs

  • Magma
    Reverse(Intseq(2^2281-1));
    
  • Mathematica
    RealDigits[2^2281 - 1, 10, 100][[1]] (* G. C. Greubel, Oct 03 2017 *)
  • PARI
    eval(Vec(Str(2^2281-1)))

Formula

Equals 2^A000043(17) - 1.
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