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 A219023 Number of primes p
 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 2, 0, 1, 1, 0, 0, 2, 1, 0, 2, 0, 0, 0, 2, 1, 1, 0, 2, 1, 0, 2, 3, 0, 2, 2, 0, 1, 4, 1, 2, 1, 0, 0, 3, 1, 1, 3, 0, 0, 1, 2, 1, 1, 1, 1, 0, 0, 2, 3, 1, 0, 3, 1, 2, 1, 0, 1, 4, 0, 1, 2, 0, 2, 3, 0, 0, 4, 0, 2, 2, 0, 1, 3, 2, 1, 4, 1, 1, 3, 3, 2, 3, 1, 2, 1, 0, 2, 4, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,12 COMMENTS Conjecture: a(n)>0 for all n>2732. We have verified this conjecture for n up to 1.4*10^7. Note that the conjecture is stronger than Oppermann's conjecture which states that for any integer n>1 both of the two intervals (n^2-n,n^2) and (n^2,n^2+n) contain primes. Zhi-Wei Sun also made the following conjectures: For n>3512 there is a prime p in (n,2n) such that both n^2-n+p and n^2+n-p are prime. For n>1828 there is a prime p4517 there is a prime in (n,2n) such that both n^2-n-p and n^2+n+p are prime. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..20000 Zhi-Wei Sun, Conjectures involving primes and quadratic forms, arXiv preprint arXiv:1211.1588, 2012. Wikipedia, Oppermann's Conjecture EXAMPLE a(12)=2 since the 5 and 7 are the only primes p<12 with 12^2-12+p and 12^2+12-p both prime. MATHEMATICA a[n_]:=a[n]=Sum[If[PrimeQ[n^2-n+Prime[k]]==True&&PrimeQ[n^2+n-Prime[k]]==True, 1, 0], {k, 1, PrimePi[n-1]}] Do[Print[n, " ", a[n]], {n, 1, 20000}] Table[Total[Table[If[AllTrue[{k^2-k+p, k^2+k-p}, PrimeQ], 1, 0], {p, Prime[ Range[ PrimePi[k]]]}]], {k, 100}] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Dec 23 2017 *) PROG (PARI) A219023(n)={my(c=0, nm=n^2-n, np=n^2+n); forprime(p=1, n-1, isprime(np-p) && isprime(nm+p) && c++); c} \\ - M. F. Hasler, Nov 11 2012 CROSSREFS Cf. A000040. Sequence in context: A281116 A089233 A066620 * A025427 A245963 A291375 Adjacent sequences:  A219020 A219021 A219022 * A219024 A219025 A219026 KEYWORD nonn AUTHOR Zhi-Wei Sun, Nov 10 2012 STATUS approved

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Last modified June 16 12:38 EDT 2019. Contains 324152 sequences. (Running on oeis4.)