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A061644 "Right perfect numbers": primes of the form 1 + a perfect number. 4
7, 29, 33550337, 137438691329 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Readers of Rivera's web page (which I believe was indirectly based on this entry) later showed that there are no more cases among the first 39 perfect numbers. - N. J. A. Sloane, May 25 2004. The latest news is that there are no more cases among the first 44 perfect numbers. - M. F. Hasler, Jun 05 2008

So of the 44 known perfect numbers P=2^(p-1)*(2^p-1), P+1 is only prime for p=2,3,13 and 19.

If p is in the sequence then for each positive integer k, p^k is a solution to the equation sigma(phi(x)) = 2x-2. Proof: take t=2 in theorem related to the sequence A093034. [M. F. Hasler and Farideh Firoozbakht, Sep 09 2014]

LINKS

Table of n, a(n) for n=1..4.

Mersenne Forum, Thread 10336

C. Rivera, Puzzle 203

FORMULA

P(p)*[P(p)+1]/2 + 1 is prime, where P(p) is a Mersenne prime. (corrected by Lekraj Beedassy, May 01 2009)

MATHEMATICA

pn={6, 28, 496, 8128, 33550336, 8589869056, 137438691328, 2305843008139952128, 2658455991569831744654692615953842176, 191561942608236107294793378084303638130997321548169216}; lst={}; Do[p=pn[[n]]+1; If[PrimeQ[p], AppendTo[lst, p]], {n, Length[pn]}]; lst... and/or...PerfectNum[n_]:=Plus@@Divisors[n]/2; lst={}; Do[p=PerfectNum[n]; If[p==n&&PrimeQ[p+1], AppendTo[lst, p+1]], {n, 10!}]; lst (* Vladimir Joseph Stephan Orlovsky, Jan 27 2009 *)

PROG

(PARI) forprime(p=2, 100, P=2^p-1; Q=P*(P+1)/2+1; if(isprime(P)&&isprime(Q), print1(Q, ", "))) \\ Edward Jiang, Sep 10 2014

CROSSREFS

Cf. A000396.

Analogous right and left multiple perfect numbers are in A093034, A094467.

Sequence in context: A122119 A300528 A157422 * A053621 A210107 A266604

Adjacent sequences:  A061641 A061642 A061643 * A061645 A061646 A061647

KEYWORD

more,nonn

AUTHOR

Labos Elemer, Jun 14 2001

STATUS

approved

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Last modified May 24 17:09 EDT 2019. Contains 323533 sequences. (Running on oeis4.)