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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
OFFSET
1,3
COMMENTS
This is taking prime(0)=1 (see first comment in A023515). - Vincenzo Librandi, Apr 27 2019
LINKS
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
KEYWORD
nonn
STATUS
approved