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A218086 Mersenne prime exponents of prime index equal to 1 or another Mersenne prime exponent. 1
2, 3, 5, 17, 127 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

No others < 24036584 (see A000043, Mersenne exponents).

More formally: {n in N | 0 < d(2^n - 1) < 3 and 0 < d(2^Pi(n) - 1) < 3}; d(n) the divisor count function and Pi(n) the prime counting function.

To n = 4, this sequence = |A218386(n) - A215929(n)| = |{2, 5, 19, 257, 196687} - {0, 2, 24, 240, 196560}|

Conjecture: This sequence is complete.

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

Pi(2) = 1

Pi(3) = 2

Pi(5) = 3

Pi(17) = 7

Pi(127) = 31

{2, 3, 5, 17, 127} are Mersenne prime exponents.

{1, 2, 3, 7, 31} are Mersenne prime exponents at the beginning of the 20th century. (see A008578, noncomposite numbers)

CROSSREFS

Cf. A218386, A215929

Sequence in context: A089983 A072858 A276629 * A291049 A087911 A265426

Adjacent sequences:  A218083 A218084 A218085 * A218087 A218088 A218089

KEYWORD

nonn

AUTHOR

Raphie Frank, Oct 20 2012

STATUS

approved

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Last modified June 22 21:29 EDT 2021. Contains 345393 sequences. (Running on oeis4.)