1,2

The cardinality (count, enumeration) of these through n equals n - card{squarefree numbers =< n} - card{trianglefree numbers =< n} + card{numbers =<n which are both square and triangular} = n - card{numbers =<n in A005117} - card{numbers =<n in A112886} + card{numbers =<n in A001110}. "There is no known polynomial time algorithm for recognizing squarefree integers or for computing the squarefree part of an integer. In fact, this problem may be no easier than the general problem of integer factorization (obviously, if an integer can be factored completely, is squarefree iff it contains no duplicated factors). This problem is an important unsolved problem in number theory" [Weisstein]. Conjecture: there is no polynomial time algorithm for recognizing numbers which are both squarefree and triangle-free.

Bellman, R. and Shapiro, H. N. "The Distribution of Squarefree Integers in Small Intervals." Duke Math. J. 21, 629-637, 1954.

Borwein, J. and Bailey, D. Mathematics by Experiment: Plausible Reasoning in the 21st Century. Natick, MA: A. K. Peters, 2003.

Hardy, G. H. and Wright, E. M. "The Number of Squarefree Numbers." Section 18.6 in An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Clarendon Press, pp. 269-270, 1979.

Eric Weisstein's World of Mathematics, Squarefree.

a(n) has no factor >1 of form a*(a+1)/2 nor b^2. A005117 INTERSECTION A112886.

The Mathematica function SquareFreeQ[n] in the Mathematica add-on package NumberTheory`NumberTheoryFunctions` (which can be loaded with the command <<NumberTheory`) determines whether a number is squarefree.

Cf. A000217, A005117, A113502, A013929, A046098, A059956, A065474, A071172, A087618, A088454, A112886.

Sequence in context: A079933 A075610 A057922 this_sequence A004134 A157829 A020581

Adjacent sequences: A113540 A113541 A113542 this_sequence A113544 A113545 A113546

easy,nonn

Jonathan Vos Post (jvospost3(AT)gmail.com), Jan 13 2006