

A005235


Fortunate numbers: least m > 1 such that m + prime(n)# is prime, where p# denotes the product of all primes <= p.
(Formerly M2418)


53



3, 5, 7, 13, 23, 17, 19, 23, 37, 61, 67, 61, 71, 47, 107, 59, 61, 109, 89, 103, 79, 151, 197, 101, 103, 233, 223, 127, 223, 191, 163, 229, 643, 239, 157, 167, 439, 239, 199, 191, 199, 383, 233, 751, 313, 773, 607, 313, 383, 293, 443, 331, 283, 277, 271, 401, 307, 331
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OFFSET

1,1


COMMENTS

Reo F. Fortune conjectured that a(n) is always prime.
You might be searching for Fortunate Primes, which is an alternative name for this sequence. It is not the official name yet, because it is possible, although unlikely, that not all the terms are primes.  N. J. A. Sloane, Sep 30 2020
The first 500 terms are primes.  Robert G. Wilson v. The first 2000 terms are primes.  Joerg Arndt, Apr 15 2013
The strong form of Cramér's conjecture implies that a(n) is a prime for n > 1618, as previously noted by Golomb.  Charles R Greathouse IV, Jul 05 2011
a(n) is the smallest m such that m > 1 and A002110(n) + m is prime. For every n, a(n) must be greater than prime(n+1)  1.  Farideh Firoozbakht, Aug 20 2003
If a(n) < prime(n+1)^2 then a(n) is prime. According to Cramér's conjecture a(n) = O(prime(n)^2).  Thomas Ordowski, Apr 09 2013
From Pierre CAMI, Sep 08 2017: (Start)
If a(n) = prime(i), lim_{N>inf} (Sum_{n=1..N} i) / (Sum_{n=1..N} n) = 3/2.
i/n is always < 6.
Lim_{N>inf} (Sum_{n=1..N} a(n)) / (Sum_{n=1..N} prime(n)) = Pi/2.
a(n) / prime(n) is always < 8.
(End)
The name "Fortunate numbers" was coined by Golomb (1981) after the New Zealand social anthropologist Reo Franklin Fortune (1903  1979). According to Golomb, Fortune's conjecture first appeared in print in Martin Gardner's Mathematical Games column in 1980.  Amiram Eldar, Aug 25 2020


REFERENCES

Martin Gardner, The Last Recreations (1997), pp. 194195.
Richard K. Guy, Unsolved Problems in Number Theory, 3rd ed., Springer, 1994, Section A2, p. 11.
Stephen P. Richards, A Number For Your Thoughts, 1982, p. 200.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
David Wells, Prime Numbers: The Most Mysterious Figures In Math, Hoboken, New Jersey: John Wiley & Sons (2005), pp. 108109.


LINKS

Pierre CAMI, Table of n, a(n) for n = 1..3000 (first 2000 terms from T. D. Noe)
Ray Abrahams and Huon Wardle, Fortune's 'Last Theorem', Cambridge Anthropology, Vol. 23, No. 1 (2002), pp. 6062.
Cyril Banderier, Conjecture checked for n < 1000 [It has been reported that the data given here contains several errors]
C. K. Caldwell, Fortunate number, The Prime Glossary.
Martin Gardner, Patterns in primes are a clue to the strong law of sma11 numbers, Mathematical Games, Scientific American, Vol. 243, No. 6 (December, 1980), pp. 1828.
Solomon W. Golomb, The evidence for Fortune's conjecture, Mathematics Magazine, Vol. 54, No. 4 (1981), pp. 209210.
Richard K. Guy, Letter to N. J. A. Sloane, 1987
Richard K. Guy, The strong law of small numbers. Amer. Math. Monthly 95 (1988), no. 8, 697712.
Richard K. Guy, The strong law of small numbers. Amer. Math. Monthly 95 (1988), no. 8, 697712. [Annotated scanned copy]
Bill McEachen, McEachen Conjecture
Romeo Meštrović, Euclid's theorem on the infinitude of primes: a historical survey of its proofs (300 BC2012) and another new proof, arXiv preprint arXiv:1202.3670 [math.HO], 2012.
Eric Weisstein's World of Mathematics, Fortunate Prime
Robert G. Wilson, V, Letter to N. J. A. Sloane with attachment, Jan 1992


FORMULA

If x(n) = 1 + Product_{i=1..n} prime(i), q(n) = least prime > x(n), then a(n) = q(n)  x(n) + 1.
a(n) = 1 + the difference between the nth primorial plus one and the next prime.
a(n) = A035345(n)  A002110(n).  Jonathan Sondow, Dec 02 2015


EXAMPLE

a(4) = 13 because P_4# = 2*3*5*7 = 210, plus one is 211, the next prime is 223 and the difference between 210 and 223 is 13.


MAPLE

Primorial:= 2:
p:= 2:
A[1]:= 3:
for n from 2 to 100 do
p:= nextprime(p);
Primorial:= Primorial * p;
A[n]:= nextprime(Primorial+p+1)Primorial;
od:
seq(A[n], n=1..100); # Robert Israel, Dec 02 2015


MATHEMATICA

NPrime[n_Integer] := Module[{k}, k = n + 1; While[! PrimeQ[k], k++]; k]; Fortunate[n_Integer] := Module[{p, q}, p = Product[Prime[i], {i, 1, n}] + 1; q = NPrime[p]; q  p + 1]; Table[Fortunate[n], {n, 60}]
r[n_] := (For[m = (Prime[n + 1] + 1)/2, ! PrimeQ[Product[Prime[k], {k, n}] + 2 m  1], m++]; 2 m  1); Table[r[n], {n, 60}]
FN[n_] := Times @@ Prime[Range[n]]; Table[NextPrime[FN[k] + 1]  FN[k], {k, 60}] (* Jayanta Basu, Apr 24 2013 *)
NextPrime[#]#+1&/@(Rest[FoldList[Times, 1, Prime[Range[60]]]]+1) (* Harvey P. Dale, Dec 15 2013 *)


PROG

(PARI) a(n)=my(P=prod(k=1, n, prime(k))); nextprime(P+2)P \\ Charles R Greathouse IV, Jul 15 2011; corrected by JeanMarc Rebert, Jul 28 2015
(Haskell)
a005235 n = head [m  m < [3, 5 ..], a010051'' (a002110 n + m) == 1]
 Reinhard Zumkeller, Apr 02 2014
(Sage)
def P(n): return prod(nth_prime(k) for k in range(1, n + 1))
it = (P(n) for n in range(1, 31))
print([next_prime(Pn + 2)  Pn for Pn in it]) # F. Chapoton, Apr 28 2020


CROSSREFS

Cf. A046066, A002110, A006862, A035345, A035346, A055211, A129912, A010051, A005408, A038771, A038711.
Sequence in context: A051507 A173145 A060274 * A107664 A321671 A085013
Adjacent sequences: A005232 A005233 A005234 * A005236 A005237 A005238


KEYWORD

nonn,nice


AUTHOR

N. J. A. Sloane


STATUS

approved



