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A258455 Divisorial primes: primes p of the form p = 1 + Product_{d|k} d for some k. 8
2, 3, 37, 101, 197, 677, 5477, 8837, 17957, 21317, 42437, 98597, 106277, 148997, 217157, 331777, 401957, 454277, 1196837, 1378277, 1674437, 1705637, 1833317, 1865957, 2390117, 2735717, 3118757, 3147077, 3587237, 3865157, 4104677, 4519877, 4726277, 5410277, 6728837 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Primes p of the form p = A007955(k) + 1 for some k.

This sequence is a sorted version of A118370.

Corresponding values of k are in A118369.

Conjectures:

(1) if 1+ Product_{d|k} d for k > 2 is a prime p, then p-1 is a square.

(2) except for n = 2, a(n) - 1 are squares.

(3) subsequence of A062459 (primes of form x^2 + mu(x)).

From Robert Israel, Jun 08 2015: (Start)

The first n > 4 for which a(n) does not end in 7 is a(918) = 34188010001.

Statements (1) and (2) are true.

Note that if k = p_1^(a_1) ... p_m^(a_m) is the prime factorization of k, then A007955(k) = p_1^(a_1*M/2) ... p_m^(a_m*M/2) where M = (a_1+1)*...*(a_m+1).  Now if M has any odd factor r > 1, A007955(k) = x^r for some x > 1 and then p = A007955(k)+1 is divisible by x+1.  So for p to be prime, M must be a power of 2.

  Now if A007955(k) is not a square, we need M/2 to be odd, so M = 2.  That can only happen if m=1 and a_1=1.  For p to be odd we need k to be even, so this means p_1 = 1, and then k=2. (End)

Union of prime 3 (where A007955(3-1) is not a square), A258896 (primes p such that p-1 = A007955(sqrt(p-1)) and A258897 (primes p such that p-1 = A007955(k) for some k < sqrt(p-1)). - Jaroslav Krizek, Jun 14 2015

Contrary to the above, this is not a subsequence of A062459: 24^4+1 = 331777 is in this sequence but not A062459. - Charles R Greathouse IV, Sep 22 2015

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..10000

EXAMPLE

The prime 37 is in sequence because there is n = 6 with divisors 1, 2, 3, 6 such that 6*3*2*1 + 1 = 37.

MAPLE

N:= 10^8: # to get all terms <= N

K:= floor(sqrt(N)):

sort(convert(select(t -> t <= N and isprime(t), {2, seq(convert(numtheory:-divisors(k), `*`)+1, k=2..K, 2)}), list)); # Robert Israel, Jun 08 2015

MATHEMATICA

terms = 35; n0 = 1000; Clear[f]; f[nmax_] := f[nmax] = Reap[For[n = 1, n <= nmax, n++, If[PrimeQ[p = Times @@ Divisors[n] + 1], Sow[p]]]][[2, 1]] // Sort // Take[#, terms]&; f[n0]; f[nmax = 2*n0]; While[f[nmax] != f[nmax/2], Print[nmax]; nmax = 2*nmax]; f[nmax] (* Jean-Fran├žois Alcover, May 31 2015 *)

Take[Sort[Select[Table[Times@@Divisors[n]+1, {n, 3000}], PrimeQ]], 40] (* Harvey P. Dale, Apr 18 2018 *)

PROG

(MAGMA) Set(Sort([&*(Divisors(n))+1: n in [1..1000000] | IsPrime(&*(Divisors(n))+1)]))

(PARI) list(lim)=my(v=List()); lim\=1; for(n=1, sqrtint(lim-1), my(d=divisors(n), t=prod(i=2, #d, d[i])+1); if(t<=lim && isprime(t), listput(v, t))); Set(v) \\ Charles R Greathouse IV, Jun 08 2015

CROSSREFS

Cf. A007955, A048943, A118369, A118370, A174895.

Sequence in context: A216145 A109748 A062459 * A118370 A189027 A061576

Adjacent sequences:  A258452 A258453 A258454 * A258456 A258457 A258458

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, May 30 2015

STATUS

approved

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Last modified December 10 05:49 EST 2018. Contains 318044 sequences. (Running on oeis4.)