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 A241566 Number of 2-element subsets of {1,...,n} whose sum has more than 2 divisors. 1

%I #4 Apr 25 2014 21:18:19

%S 0,0,1,2,5,8,12,17,22,27,34,41,50,60,70,80,92,105,119,134,149,164,181,

%T 198,216,235,254,274,296,318,341,365,390,415,441,467,494,522,551,580,

%U 611,642,675,709,743,778,815,853,891,930,969,1008,1049,1090,1131

%N Number of 2-element subsets of {1,...,n} whose sum has more than 2 divisors.

%C If the constraint on the number of divisors is dropped, one gets A000217 = triangular numbers n*(n+1)/2, which therefore is an upper bound.

%C If one considers 3-element subsets instead, one gets A241563.

%o (PARI) a(n,m=2,d=2)={s=0;u=vector(m,n,1)~;forvec(v=vector(m,i,[1,n]),numdiv(v*u)>d&&s++,2);s}

%K nonn

%O 1,4

%A _M. F. Hasler_, Apr 25 2014

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Last modified August 9 02:18 EDT 2024. Contains 375024 sequences. (Running on oeis4.)