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A217836 a(n) is largest semiprime < 2*a(n-1), with a(1) = 4. 1

%I

%S 4,6,10,15,26,51,95,187,371,737,1473,2942,5878,11755,23507,47013,

%T 94021,188041,376069,752135,1504261,3008503,6017001,12034001,24068001,

%U 48135995,96271987,192543973,385087943,770175883,1540351763,3080703523,6161407045,12322814089,24645628171

%N a(n) is largest semiprime < 2*a(n-1), with a(1) = 4.

%C This is to Bertrand primes A006992 as semiprimes A001358 are to primes A000040.

%H Robert G. Wilson v, <a href="/A217836/b217836.txt">Table of n, a(n) for n = 1...225</a>

%e a(2) = 6 because that is the largest semiprime < 2*a(1) = 8, where a(1) is the first semiprime.

%e a(3) = 10, the largest semiprime < 2*6 = 12.

%t PrevSemiPrime[n_, k_] := Block[{c = 0, sp = n - 1}, While[c < k, While[ PrimeOmega[sp] != 2, sp--]; sp--; c++]; sp + 1]; NestList[ PrevSemiPrime[ 2#, 1] &, 4, 34] (* _Robert G. Wilson v_, Oct 19 2012 *)

%Y Cf. A001358, A006992.

%K nonn,easy

%O 1,1

%A _Jonathan Vos Post_, Oct 19 2012

%E Terms greater than a(15) from _Robert G. Wilson v_, Oct 19 2012

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Last modified September 27 22:36 EDT 2020. Contains 337388 sequences. (Running on oeis4.)