A317307 - OEIS

A317307

Sum of divisors of powers of 2 and sum of divisors of even perfect numbers.

1, 3, 7, 12, 15, 31, 56, 63, 127, 255, 511, 992, 1023, 2047, 4095, 8191, 16256, 16383, 32767, 65535, 131071, 262143, 524287, 1048575, 2097151, 4194303, 8388607, 16777215, 33554431, 67100672, 67108863, 134217727, 268435455, 536870911, 1073741823, 2147483647, 4294967295, 8589934591, 17179738112, 17179869183

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OFFSET

1,2

COMMENTS

Sum of divisors of the numbers k such that the symmetric representation of sigma(k) has only one part, and apart from the central width, the rest of the widths are 1's.

Note that the above definition implies that the central width of the symmetric representation of sigma(k) is 1 or 2. For powers of 2 the central width is 1. For even perfect numbers the central width is 2 (see example).

LINKS

Table of n, a(n) for n=1..40.

FORMULA

a(n) = A000203(A317306(n)).

EXAMPLE

Illustration of initial terms. a(n) is the area (or the number of cells) of the n-th region of the diagram:

. _ _ _ _ _ _ _ _

. 1 |_| | | | | | | | | | | | | |

. 3 |_ _|_| | | | | | | | | | | |

. _ _| _|_| | | | | | | | | |

. 7 |_ _ _| _|_| | | | | | | |

. _ _ _| _| _ _| | | | | | |

. 12 |_ _ _ _| _| | | | | | |

. _ _ _ _| | | | | | | |

. 15 |_ _ _ _ _| _ _ _| | | | | |

. | _ _ _| | | | |

. _| | | | | |

. _| _| | | | |

. _ _| _| | | | |

. | _ _| | | | |

. | | _ _ _ _ _| | | |

. _ _ _ _ _ _ _ _| | | _ _ _ _ _| | |

. 31 |_ _ _ _ _ _ _ _ _| | | _ _ _ _ _ _| |

. _ _| | | _ _ _ _ _ _|

. _ _| _ _| | |

. | _| _ _| |

. _| _| | _ _|

. | _| _| |

. _ _ _| | _| _|

. | _ _ _| _ _| _|

. | | | _ _|

. | | _ _ _| |

. | | | _ _ _|

. _ _ _ _ _ _ _ _ _ _ _ _ _ _| | | |

. 56 |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _| | |

. | |

. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _| |

. 63 |_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|

The diagram shows the first eight terms of the sequence. The symmetric representation of sigma of the numbers A317306: 1, 2, 4, 6, 8, 16, 28, 32, ..., has only one part, and apart from the central width, the rest of the widths are 1's.

MATHEMATICA

DivisorSigma[1, #] &@ Union[2^Range[0, Floor@ Log2@ Last@ #], #] &@ Array[2^(# - 1) (2^# - 1) &@ MersennePrimeExponent@ # &, 7] (* Michael De Vlieger, Aug 25 2018, after Robert G. Wilson v at A000396 *)

CROSSREFS

Union of nonzero terms of A000225 and A139256.

Odd terms give the nonzeros terms of A000225.

Even terms give A139256.

Subsequence of A317305.

Cf. A249351 (the widths).

Cf. A000203, A000396, A196020, A236104, A235791, A237048, A237591, A237593, A237270, A237271, A239660, A239931, A239932, A239933, A239934, A244050, A245092, A262626, A317306.

Sequence in context: A356425 A096998 A356353 * A331042 A122981 A122982

Adjacent sequences: A317304 A317305 A317306 * A317308 A317309 A317310

KEYWORD

nonn,easy

AUTHOR

Omar E. Pol, Aug 25 2018

STATUS

approved