

A103981


Number of prime factors (with multiplicity) of octahedral numbers (A005900).


1



0, 0, 2, 1, 3, 2, 2, 3, 4, 2, 3, 5, 4, 2, 3, 3, 7, 2, 4, 2, 5, 2, 4, 2, 4, 4, 4, 3, 4, 4, 3, 2, 6, 2, 4, 4, 4, 3, 5, 3, 6
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OFFSET

0,3


COMMENTS

When a(n) = 2, n is an element of A103982: indices of octahedral numbers (A005900) which are semiprimes.


REFERENCES

Conway, J. H. and Guy, R. K. The Book of Numbers. New York, SpringerVerlag, p. 50, 1996
Dickson, L. E. History of the Theory of Numbers, Vol. 2: Diophantine Analysis. New York: Chelsea, 1952.


LINKS

Table of n, a(n) for n=0..40.
H. K. Kim, On Regular Polytope Numbers, Journal: Proc. Amer.Math. Soc. 131 (2003), 6575, as PDF file.
Eric Weisstein's World of Mathematics, Octahedral Number


FORMULA

a(n) = A001222(A005900(n)). a(n) = Bigomega((2*n^3 + n)/3).


EXAMPLE

a(3) = 1 because OctahedralNumber(3) = A005900(3) = 19, which is prime and thus has only one prime factor. Because the cubic polynomial for octahedral numbers factors into n time a quadratic, the octahedral numbers can never be prime after a(3) = 19.
a(4) = 3 because A005900(4) = (2*4^3 + 4)/3 = 44 = 2 * 2 * 11, which has (with multiplicity) three prime factors.


CROSSREFS

Cf. A001222, A005900, A103946, A103982.
Sequence in context: A237130 A058773 A122805 * A029270 A090350 A199133
Adjacent sequences: A103978 A103979 A103980 * A103982 A103983 A103984


KEYWORD

easy,nonn


AUTHOR

Jonathan Vos Post, Feb 24 2005


STATUS

approved



