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%I #15 Mar 01 2020 06:30:33
%S 1,33,65,81,121,129,141,201,213,231,243,279,309,325,345,361,385,451,
%T 475,531,543,585,605,629,651,697,713,755,775,807,831,901,965,1001,
%U 1029,1261,1309,1329,1395,1495,1565,1589,1661,1737,1773,1833,1857,1935,1967
%N Lengths of Farey-series (A005728) which are not primes.
%H Amiram Eldar, <a href="/A061560/b061560.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A005728(A055197(n-1)) for n > 1, a(1) = A005728(0).
%e a(3) = 65 = 5*13 is not a prime, A055197(3-1) = 14, A005728(14) = 65.
%t seq = {1}; sum = 1; Do[sum += EulerPhi[k]; If[!PrimeQ[sum], AppendTo[seq, sum]], {k, 1, 80}]; seq (* _Amiram Eldar_, Mar 01 2020 *)
%Y Cf. A005728, A055197, A006842, A006843, A002088, A000010, A055201.
%K nonn
%O 1,2
%A _Frank Ellermann_, May 17 2001
%E More terms from _Vladeta Jovovic_, Jun 03 2001
%E Offset corrected by _Amiram Eldar_, Mar 01 2020
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