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A270992 Number of distinct prime divisors of prime(n)+1 and prime(n+1)+1. 2
0, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,9

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

For n=1, p=2 and q=3; 3 and 4 have no common prime divisor, so a(1)=0.

For n=2, p=3 and q=5; 4 and 6 have 1 common prime divisor, so a(2)=1.

For n=9, p=23 and q=29; 24 and 30 have 2 common prime divisors, so a(9)=2.

MATHEMATICA

Table[Length[Map[First, FactorInteger[GCD @@ {Prime@ n + 1, Prime[n + 1] + 1}]] /. 1 -> Nothing], {n, 101}] (* Michael De Vlieger, Mar 28 2016 *)

Length[Intersection[FactorInteger[#[[1]]+1][[All, 1]], FactorInteger[#[[2]] + 1][[All, 1]]]]&/@Partition[Prime[Range[120]], 2, 1] (* Harvey P. Dale, Jun 11 2017 *)

PROG

(PARI) lista(nn) = {p = 2; f = factor(p+1)[, 1]~; forprime(q=3, nn, g = factor(q+1)[, 1]~; print1(#setintersect(f, g), ", "); p = q; f = g; ); }

(PARI) a(n) = my(p = prime(n), q = nextprime(p+1)); #setintersect(factor(p+1)[, 1]~, factor(q+1)[, 1]~);

CROSSREFS

Cf. A008335, A270592 (records).

Sequence in context: A211993 A185646 A037829 * A117546 A274196 A096811

Adjacent sequences:  A270989 A270990 A270991 * A270993 A270994 A270995

KEYWORD

nonn

AUTHOR

Michel Marcus, Mar 28 2016

STATUS

approved

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Last modified September 25 06:32 EDT 2020. Contains 337335 sequences. (Running on oeis4.)