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a(n) = 2^ceiling(n*(n+1)/3).
1

%I #18 Sep 08 2022 08:45:54

%S 1,2,4,16,128,1024,16384,524288,16777216,1073741824,137438953472,

%T 17592186044416,4503599627370496,2305843009213693952,

%U 1180591620717411303424,1208925819614629174706176,2475880078570760549798248448

%N a(n) = 2^ceiling(n*(n+1)/3).

%C Hankel transform of A128750. Satisfies (4,0) Somos-4 recurrence 4*a(n-1)*a(n-3)/a(n-4) = a(n),n>3.

%H G. C. Greubel, <a href="/A179532/b179532.txt">Table of n, a(n) for n = 0..99</a>

%F 2^A007980.

%F a(n) = a(-1-n) for all n in Z. - _Michael Somos_, Aug 14 2018

%t Table[2^Ceiling[(n(n+1))/3],{n,0,20}] (* _Harvey P. Dale_, Sep 25 2013 *)

%o (PARI) vector(20,n,n--; 2^ceil(n*(n+1)/3)) \\ _G. C. Greubel_, Aug 14 2018

%o (Magma) [2^Ceiling((n(n+1))/3): n in [0..20]]; // _G. C. Greubel_, Aug 14 2018

%K nonn

%O 0,2

%A _Paul Barry_, Jan 08 2011