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A023516 Number of distinct prime divisors of prime(n)*prime(n-1) - 1. 1
0, 1, 2, 2, 2, 2, 3, 3, 2, 3, 2, 3, 2, 2, 3, 4, 3, 3, 3, 3, 2, 3, 3, 3, 3, 3, 3, 4, 3, 2, 4, 2, 3, 2, 4, 3, 3, 4, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 4, 2, 3, 3, 4, 4, 4, 4, 4, 3, 4, 2, 3, 4, 2, 4, 4, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 4, 3, 2, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

This is taking prime(0)=1 (see first comment in A023515). - Vincenzo Librandi, Apr 27 2019

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A001221(A023515(n)).

MAPLE

0, seq(nops(numtheory:-factorset(ithprime(n)*ithprime(n-1)-1)), n=2..120); # Muniru A Asiru, Apr 29 2019

MATHEMATICA

Prepend[Table[PrimeNu[Prime[n] Prime[n-1] - 1], {n, 2, 80}], 0] (* Vincenzo Librandi, Apr 27 2019 *)

PROG

(MAGMA) [#PrimeDivisors(NthPrime(n)*(NthPrime(n-1))-1): n in [1..100]]; // Vincenzo Librandi, Apr 27 2019

(PARI) a(n) = if (n==1, 0, omega(prime(n)*prime(n-1) - 1)); \\ Michel Marcus, Apr 30 2019

CROSSREFS

Cf. A023515, A023524, A023568, A023575, A023582, A023589.

Sequence in context: A247069 A322168 A118377 * A156607 A093450 A096198

Adjacent sequences:  A023513 A023514 A023515 * A023517 A023518 A023519

KEYWORD

nonn

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified May 24 04:40 EDT 2022. Contains 354005 sequences. (Running on oeis4.)