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a(n) is the least semiprime that starts with n.
2

%I #11 Jul 23 2024 20:41:40

%S 10,21,33,4,51,6,74,82,9,10,111,121,133,14,15,161,177,183,194,201,21,

%T 22,235,247,25,26,274,287,291,301,314,321,33,34,35,361,371,38,39,403,

%U 411,422,437,445,451,46,471,481,49,501,51,526,533,542,55,562,57,58,591,6001,611,62,633,649,65,662

%N a(n) is the least semiprime that starts with n.

%H Robert Israel, <a href="/A374899/b374899.txt">Table of n, a(n) for n = 1..10000</a>

%e a(5) = 51 because 51 = 3 * 17 is a semiprime that starts with 5, and is the least such semiprime.

%p f:= proc(n) local d,x;

%p if numtheory:-bigomega(n)=2 then return n fi;

%p for d from 1 do

%p for x from n*10^d to (n+1)*10^d-1 do

%p if numtheory:-bigomega(x)=2 then return x fi

%p od od

%p end proc:

%p map(f, [$1..100]);

%Y Cf. A001358, A018800, A374897.

%K nonn,base,look

%O 1,1

%A _Robert Israel_, Jul 22 2024