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A108728 Number of distinct prime divisors of concatenated even numbers. 3
1, 2, 3, 2, 5, 3, 2, 3, 6, 5, 3, 4, 6, 4, 5, 4, 3, 5, 4, 7, 6, 5, 8, 3, 7, 4, 4, 8, 5, 7, 4, 4, 8, 5, 7, 6, 4, 8, 8, 7, 5, 3, 7, 4, 6, 10, 11, 5, 4, 10, 6, 5, 6, 5, 5, 4, 9, 4, 8, 9, 6, 5, 8, 4, 12, 5, 4, 8, 10, 5, 9, 7, 6, 8, 8, 5, 10, 5, 9, 7, 5, 5, 8, 3, 11, 6, 6, 7, 9 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
LINKS
Apurva Rai and Michael S. Branicky, Prime Factorizations of OEIS A019520
FORMULA
a(n) = A001221(A019520(n)). - Amiram Eldar, Jan 27 2020
EXAMPLE
Number of distinct prime divisors of 2 is 1 - which is the first term.
Number of distinct prime divisors of 24 is 2 - which is the second term.
Number of distinct prime divisors of 246 is 3 - which is the third term.
MATHEMATICA
a[n_] := PrimeNu @ FromDigits @ Flatten[IntegerDigits /@ (2*Range[n])]; Array[a, 30] (* Amiram Eldar, Jan 27 2020 *)
CROSSREFS
Cf. A001221, A019520, A105388 (number of divisors).
Sequence in context: A054494 A306252 A112764 * A331962 A302170 A049805
KEYWORD
nonn,base
AUTHOR
Parthasarathy Nambi, Jun 21 2005
EXTENSIONS
a(43)-a(52) from Amiram Eldar, Jan 27 2020
a(53)-a(54) from Jinyuan Wang, Jun 27 2020
a(55)-a(69) from Apurva Rai and Michael S. Branicky, Aug 16 2020
a(70)-a(71) from Max Alekseyev, Mar 21 2023
a(72)-a(89) from Tyler Busby, Mar 23 2023
STATUS
approved

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Last modified March 28 16:58 EDT 2024. Contains 371254 sequences. (Running on oeis4.)