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A059411 a(1) = 2, a(n) = k*a(n-1) + 1, where a(n) is the smallest prime of the form k*a(n-1) + 1 and k > 1. 7
2, 5, 11, 23, 47, 283, 1699, 20389, 244669, 7340071, 205521989, 411043979, 4932527749, 295951664941, 4735226639057, 227290878674737, 12273707448435799, 883706936287377529, 24743794216046570813 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A sequence of primes generated recursively as follows: a(n+1) = q(n)*a(n)+1, where q=q(n) is the smallest (even) number such that a(n+1) = q*a(n)+1 is prime and the initial value a(1)=2. q(n) = (a(n+1) - 1)/a(n) is the satellite "almost-quotient-sequence".

It has been established in the Murthy reference that for every prime p there exists at least one prime of the form k*p +1. Hence the sequence is infinite. - Amarnath Murthy, Mar 02 2002

The existence of a prime of the form k*p+1 follows from Dirichlet's theorem (1837). - T. D. Noe, Mar 14 2009

REFERENCES

Amarnath Murthy, On the divisors of Smarandache Unary Sequence. Smarandache Notions Journal, Vol. 11, No. 1-2-3, Spring 2000.

LINKS

T. D. Noe, Table of n, a(n) for n = 1..100

FORMULA

a(n+1) = a(n)*q(n) + 1, q(n) = Min{q|qa(n)+1 is prime}.

EXAMPLE

a(5) = 47 and a(6) = 283 = 6*47 +1 is the smallest such prime.

The initial values are safe primes: (2), 5, 11, 23, 47, ... To obtain qa(i)+1 primes q > 2 multiplier arises and such a q always exists in arithmetic progression of difference a(i). E.g., {1699*2k+1} gives the first prime when 2k=12. So a(7)=1699 is followed by 1699*12+1 = 20389 = a(8). The emergent "quotient-sequence" is {2, 2, 2, 2, 6, 6, 12, 12, 30, 28, 2, 12, 60, 16, 48, 54, 72, 28, 180, 102, 4, 12, 106, 50, 18}. A059411 is an infinite sequence of primes increasing at least with exponential speed.

MAPLE

i := 0:a[0] := 2:while(i<40) do k := 2:while(not isprime(k*a[i]+1)) do k := k+1; end do; i := i+1; a[i] := k*a[i-1]+1; end do:q := seq(a[i], i=0..39);

MATHEMATICA

nxt[n_]:=Module[{k=2}, While[!PrimeQ[k*n+1], k++]; k*n+1]; NestList[nxt, 2, 20] (* Harvey P. Dale, Dec 26 2014 *)

CROSSREFS

Cf. A061092.

Sequence in context: A055011 A007505 A246492 * A126017 A292937 A034468

Adjacent sequences:  A059408 A059409 A059410 * A059412 A059413 A059414

KEYWORD

nonn

AUTHOR

Labos Elemer, Jan 30 2001

STATUS

approved

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Last modified December 8 18:37 EST 2019. Contains 329865 sequences. (Running on oeis4.)