A308339 - OEIS

%I #14 Jun 01 2019 10:14:58

%S 21,30,49,51,75,82,96,106,120,130,133,136,141,148,152,157,161,173,177,

%T 180,186,187,189,202,207,209,213,217,221,226,236,240,242,244,248,261,

%U 264,277,285,286,294,305,306,311,317,320,322,327,333,349,355,364,368

%N Sphenic number indices of A215217.

%C A215217 is a subsequence of A007304 (by definition). The index sequence is more compact.

%H Peter Dolland, <a href="/A308339/b308339.txt">Table of n, a(n) for n = 1..200</a>

%F A215217(n) = A007304(a(n)).

%e For n = 3: a(3) = 49 and A215217(3) = 429 = A007304(49).

%o (Haskell)

%o twinLowX [] = []

%o twinLowX [_] = []

%o twinLowX (n : (m : ns))

%o     | m == n + 1 = 1 : (map succ (twinLowX (m : ns)))

%o     | otherwise = (map succ (twinLowX (m : ns)))

%o a308339 n = (twinLowX a007304_list) !! (n - 1)

%o -- _Peter Dolland_, May 31 2019

%Y Cf. A007304, A215217.

%K nonn

%O 1,1

%A _Peter Dolland_, May 20 2019