A131866 - OEIS

%I #10 Aug 12 2016 12:32:39

%S 0,2,0,1,2,1,4,3,0,1,3,2,1,2,3,3,0,2,6,7,6,2,1,5,7,4,1,4,5,6,9,7,6,5,

%T 6,10,6,3,2,0,1,2,8,11,10,3,2,1,1,2,11,11,10,8,3,0,8,9,13,11,9,2,5,6,

%U 7,9,10,13,12,11,10,8,7,6,4,1,10,12,9,7,3,2,3,6,9,11,15,11,2,0,2,6,9,10,12

%N Distance of n-th semiprime to nearest square.

%C This to semiprimes A001358 as A047972 is to primes A000040.

%C For each semiprime, find the closest square (preceding or succeeding); subtract, take absolute value.

%H Harvey P. Dale, <a href="/A131866/b131866.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n)=A053188(A001358(n)) (corrected by R. J. Mathar, Nov 19 2007).

%e a(1) = 0 because the first semiprime is 4, which is a square.

%e a(2) = 2 because the 2nd semiprime is 6 and |6-4| = 2 where 4 is the nearest square to 6.

%e a(3) = 0 because the 3rd semiprime is 9, which is a square.

%e a(4) = 1 because the 4th semiprime is 10 and |10-9| = 1 where 9 is the nearest square to 10.

%t dns[n_]:=Min[n-Floor[Sqrt[n]]^2,Ceiling[Sqrt[n]]^2-n]; dns/@Select[ Range[ 400],PrimeOmega[#]==2&] (* _Harvey P. Dale_, Aug 12 2016 *)

%Y Cf. A001358, A047972, A053188.

%K easy,nonn

%O 1,2

%A _Jonathan Vos Post_, Oct 04 2007

%E More terms from _R. J. Mathar_, Oct 24 2007