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A255770
Number of distinct prime factors of A220161(n).
4
1, 2, 3, 4, 6, 8, 9, 11, 13, 17, 19, 21
OFFSET
0,2
COMMENTS
This strictly increasing sequence proves (yet again) the infinitude of primes.
REFERENCES
Arthur Engel, Problem-Solving Strategies, Springer, 1998, pages 121-122 (E3, said to be a "recent competition problem from the former USSR").
EXAMPLE
A220161(0) = 7 so a(0) = 1.
A220161(1) = 3*7 so a(1) = 2.
A220161(2) = 3*7*13 so a(2) = 3.
A220161(3) = 3*7*13*241 so a(3) = 4.
A220161(4) = 3*7*13*97*241*673 so a(4) = 6.
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Hans Havermann, Mar 06 2015
EXTENSIONS
Offset changed by Arkadiusz Wesolowski, Aug 01 2016
STATUS
approved