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These are necessarily the indices of record maxima in A166006.
Hans Havermann, Table of n, a(n) for n = 1..10000
Hans Havermann, Table of n, a(n) for n = 1..67011
3 first occurs in A166006 at position 21, so a(3) = 21.
Cf. A166006, A039624, A278738.
Sequence in context: A114846 A195165 A201973 * A162393 A180700 A077526
Adjacent sequences: A278734 A278735 A278736 * A278738 A278739 A278740
Hans Havermann, Nov 27 2016