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 A075432 Primes with no squarefree neighbors. 12
 17, 19, 53, 89, 97, 127, 149, 151, 163, 197, 199, 233, 241, 251, 269, 271, 293, 307, 337, 349, 379, 449, 487, 491, 521, 523, 557, 577, 593, 631, 701, 727, 739, 751, 773, 809, 811, 881, 883, 919 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Complement of A075430 in A000040. I propose a shorter name: non-Euclidean primes. That is justified by the Euclid's demonstration of the infinitude of primes. It appears that the proportion of non-Euclidean primes respect to primes tend to the limit 1-2A where A = .37395581... is Artin's constant. This table calculated by Jens K. Andersen corroborate it: 10^5: 2421 / 9592 = 0.2523978315 10^6: 19812 / 78498 = 0.2523885958 10^7: 167489 / 664579 = 0.2520227091 10^8: 1452678 / 5761455 = 0.2521373507 10^9: 12817966 / 50847534 = 0.2520862860 10^10: 114713084 / 455052511 = 0.2520875750 10^11: 1038117249 / 4118054813 = 0.2520892256 It comes close to the expected 1-2A. - Ludovicus (luiroto(AT)yahoo.com), Dec 07 2009 This sequence is infinite by Dirichlet's theorem, since there are infinitely many primes = 17 or 19 (mod 36) and these have no squarefree neighbors. Ludovicus' conjecture about density is correct. Capsule proof: either p-1 or p+1 is divisible by 4, so it suffices to consider the other number (WLOG p+1). For some fixed bound L, p is not divisible by any prime q < L (with finitely many exceptions) so there are q^2 - q possible residue classes for p. The primes in each are uniformly distributed so the probability that p+1 is divisible by q^2 is 1/(q^2 - q). The product of the complements goes to 2A as L increases without bound, and since 2A is an upper bound the limit is sandwiched between. - Charles R Greathouse IV, Aug 27 2014 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Pieter Moree, Artin's primitive root conjecture -a survey -, arXiv:math/0412262 [math.NT], 2004-2012. C. Rivera, Conjecture 65. Non-Euclidean primes FORMULA a(n) ~ Cn log n, where C = 1/(1 - 2A) = 1/(1 - prod_{p>2 prime} 1 - 1/(p^2-p)), where A is the constant in A005596. - Charles R Greathouse IV, Aug 27 2014 MAPLE filter:= n -> isprime(n) and not numtheory:-issqrfree(n+1) and not numtheory:-issqrfree(n-1): select(filter, [seq(2*i+1, i=1..1000)]); # Robert Israel, Aug 27 2014 MATHEMATICA lst={}; Do[p=Prime[n]; If[ !SquareFreeQ[Floor[p-1]] && !SquareFreeQ[Floor[p+1]], AppendTo[lst, p]], {n, 6!}]; lst (* Vladimir Joseph Stephan Orlovsky, Dec 20 2008 *) Select[Prime[Range[300]], !SquareFreeQ[#-1]&&!SquareFreeQ[#+1]&] (* Harvey P. Dale, Apr 24 2014 *) PROG (Haskell) a075432 n = a075432_list !! (n-1) a075432_list = f [2, 4 ..] where    f (u:vs@(v:ws)) | a008966 v == 1 = f ws                    | a008966 u == 1 = f vs                    | a010051' (u + 1) == 0 = f vs                    | otherwise            = (u + 1) : f vs -- Reinhard Zumkeller, May 04 2013 (PARI) is(n)=!issquarefree(if(n%4==1, n+1, n-1)) && isprime(n) \\ Charles R Greathouse IV, Aug 27 2014 CROSSREFS Cf. A039787, A049097, A005117, A000040, A008966, A010051. Sequence in context: A243437 A144709 A132239 * A232882 A119768 A232878 Adjacent sequences:  A075429 A075430 A075431 * A075433 A075434 A075435 KEYWORD nonn AUTHOR Reinhard Zumkeller, Sep 15 2002 STATUS approved

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