OFFSET
1,1
COMMENTS
a(8) > 10^7. - Tyler Busby, Feb 07 2023
a(9) > 10^8. - Daniel Suteu, Feb 08 2023
EXAMPLE
All of the following are the least semiprime with the required characteristics.
a(1) = 6 because 6 is a semiprime and its 'Look & Say' transformation A045918(6) = 16 is not a semiprime. (The smaller semiprime 4 yields LS(4)=14 which is again a semiprime.)
a(2) = 14 because both 14 and A045918(14)=1114 are semiprimes but LS(1114)=3114 is not.
a(3) = 4 because 4 (2*2), 14 (2*7) and 1114 (2*557) are all semiprimes but 3114 (2*3*3*173) is not.
MATHEMATICA
LookAndSayA[n_] := FromDigits@ Flatten@ IntegerDigits@ Flatten[ Through[ {Length, First}[#]] & /@ Split@ IntegerDigits@ n]; semiPrimeQ[n_] := Plus @@ Last /@ FactorInteger@ n == 2; f[n_] := Block[{k = 1, truth = Append[Table[True, {n}], False]}, While[ semiPrimeQ@# & /@ NestList[ LookAndSayA, k, n] != truth, k++]; k]
PROG
(PARI) A205300(n) = for(a=4, 1e9, bigomega(a)==2||next; my(t=a); for(k=2, n, bigomega(t=A045918(t))!=2 && next(2)); bigomega(A045918(t))==2 || return(a)) \\ M. F. Hasler, Jan 30 2012
CROSSREFS
KEYWORD
nonn,base,hard,more
AUTHOR
Robert G. Wilson v, Jan 27 2012
EXTENSIONS
a(7) from Tyler Busby, Feb 07 2023
a(8) from Daniel Suteu, Feb 08 2023
STATUS
approved