a(0) = 0;
a(1) = 7 = ceiling(2*Pi), a single unit circle has a circumference of 2Pi;
a(2) = 11 = ceiling(2*Pi+4), two adjacent unit circles;
a(3) = 13 = ceiling(2*Pi+6), three coins arranged in a triangle - 2 circles in one row and one circle above;
a(4) = 15 = ceiling(2*Pi+8), four circles arranged in a square or parallelogram;
a(5) = 17 = ceiling(2*Pi+10), for 5 circles, either 3 in one row and two in the other, 2,2,&1 or 5 in a circle;
a(6) = 18 = ceiling(2*pi+8+4*cos(Pi/6)), for 6 circles, see Table 1, C06 and Fig. 7 in Kallrath & Frey;
a(7) = 19 = ceiling(2*Pi+12), for 7, it is 2,3,2 pattern;
a(8) = 21 = ceiling(2*Pi+14);
a(9) = 22 = ceiling(2*Pi+12+4*cos(Pi/6));
a(10) = 23 = ceiling(2*Pi+16);
a(11) = 24 = ceiling(2*Pi+14+4*cos(Pi/6));
a(12) = 25 = ceiling(2*Pi+18);
a(13) = 26 = ceiling(2*Pi+16+4*cos(Pi/6));
a(14) = 27 = ceiling(2*Pi+20);
a(15) = 28 = ceiling(2*Pi+18+4*cos(Pi/6));
a(16) = 29 = ceiling(2*Pi+22);
a(17) = 30 = ceiling(2*Pi+20+4*cos(Pi/6));
a(18) = 31 = ceiling(2*Pi+24);
a(19) = 33 = ceiling(2*Pi+26);
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