%I A028391
%S A028391 0,0,1,2,2,3,4,5,6,6,7,8,9,10,11,12,12,13,14,15,16,17,18,
%T A028391 19,20,20,21,22,23,24,25,26,27,28,29,30,30,31,32,33,34,
%U A028391 35,36,37,38,39,40,41,42,42,43,44,45,46,47,48,49,50,51
%N A028391 a(n) = n - floor[sqrt(n)].
%C A028391 Number of non-squares <= n.
%C A028391 Number of numbers k (<=n) with an even number of divisors - Benoit Cloitre 
               (benoit7848c(AT)orange.fr), Sep 07 2002
%C A028391 Construct the pyramid
%C A028391 ............a(0)
%C A028391 .......a(1).a(2).a(3)
%C A028391 ..a(4).a(5).a(6).a(7).a(8).. etc.
%C A028391 Now circle all the primes and the result will be a pattern very similar 
               to the famous Ulam spiral. - Sam Alexander (amnalexander(AT)yahoo.com), 
               Nov 14 2003
%C A028391 The sequence floor[n-n^(1/2)] gives the same numbers with a different 
               offset. - Mohammad K. Azarian (azarian(AT)evansville.edu), R. J. 
               Mathar and M. F. Hasler, Apr 30 2008
%C A028391 The number of non-zero values of floor (j^2/n) taken over 1 <= j <= n-1.
%D A028391 B. Alspach, K. Heinrich and G. Liu, Orthogonal factorizations of graphs, 
               pp. 13-40 of Contemporary Design Theory, ed. J. H. Dinizt and D. 
               R. SAtinson, Wiley, 1992 (see Theorem 2.7).
%H A028391 Boland, Dick. <a href="http://imathination.net/SSS_first.htm">Introduction 
               to the Square Spine Spiral</a>, 2000-2003.
%F A028391 Also ceiling( n+1 - sqrt(n+1) ).
%Y A028391 Cf. A056847, A000196.
%Y A028391 Cf. A134914, A135660, A135661, A135662, A135663, A135664, A135665, A135666.
%Y A028391 Cf. A166373
%Y A028391 Sequence in context: A119353 A140859 A072586 this_sequence A135666 A038668 
               A071754
%Y A028391 Adjacent sequences: A028388 A028389 A028390 this_sequence A028392 A028393 
               A028394
%K A028391 nonn,easy,nice
%O A028391 0,4
%A A028391 John Mellor (u15630(AT)snet.net)
%E A028391 Edited by N. J. A. Sloane (njas(AT)research.att.com) at the suggestion 
               of R. J. Mathar, May 01 2008
%E A028391 Comment and cross-reference added by Christopher Hunt Gribble (chris.eveswell(AT)virgin.net), 
               Oct 13 2009