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A239708 Numbers of the form m = 2^i + 2^j, where i > j >= 0, such that m - 1 is prime. 9
3, 6, 12, 18, 20, 24, 48, 68, 72, 80, 132, 192, 258, 264, 272, 384, 1032, 1040, 1088, 1152, 1280, 2064, 2112, 4100, 4112, 4128, 4160, 5120, 6144, 8448, 16448, 20480, 32772, 32784, 32832, 33024, 33792, 65538, 65540, 65544, 65552, 65600, 66048, 73728, 81920, 262148, 262152, 262272, 262400, 263168, 266240, 294912, 524352, 528384, 786432 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Complement of the disjunction of A079696 with A187813. This means that a number m is a term if and only if b = 2 is the only base for which the base-b digital sum of m is b.

LINKS

Hieronymus Fischer, Table of n, a(n) for n = 1..250

FORMULA

A239703(a(n)) = 1.

EXAMPLE

a(1) = 3, since 3 = 2^1 + 2^0.

a(3) = 12, since 12 = 2^3 + 2^2.

PROG

(Smalltalk)

A239708

"Answers the n-th term of A239708.

  Usage: n A239708

  Answer: a(n)"

  | a b i k m p q terms |

  terms := OrderedCollection new.

  b := 2.

  p := 1.

  k := 0.

  m := 0.

  [k < self] whileTrue:

         [m := m + 1.

         p := b * p.

         q := 1.

         i := 0.

         [i < m and: [k < self]] whileTrue:

                   [i := i + 1.

                   a := p + q.

                   (a - 1) isPrime

                        ifTrue:

                            [k := k + 1.

                            terms add: a].

                   q := b * q]].

  ^terms at: self

-----------------

(Smalltalk)

A239708inv

  "Answers a kind of inverse of A239708.

  Usage: n A239708inv

  Answer: max ( k | A239708(k) < n)"

  | k |

  k := 1.

  [k A239708 < self] whileTrue: [k := k + 1].

  ^k - 1

CROSSREFS

Cf. A239709 - A239720.

Cf. A239703, A187813, A079696, A008864.

Sequence in context: A288570 A038046 A162845 * A038588 A111041 A079830

Adjacent sequences:  A239705 A239706 A239707 * A239709 A239710 A239711

KEYWORD

nonn

AUTHOR

Hieronymus Fischer, Mar 27 2014

STATUS

approved

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Last modified July 10 00:05 EDT 2020. Contains 335570 sequences. (Running on oeis4.)