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A206011
The n-th semiprime minus its sum of digits.
1
0, 0, 0, 9, 9, 9, 18, 18, 18, 18, 27, 27, 27, 27, 27, 36, 36, 45, 45, 45, 45, 54, 54, 54, 63, 63, 72, 72, 72, 72, 81, 81, 81, 81, 99, 108, 108, 108, 108, 117, 117, 117, 117, 126, 126, 135, 135, 135, 135, 135, 144, 144, 144, 153, 153, 153, 162, 162, 171, 171
OFFSET
1,4
COMMENTS
This is to semiprimes A001358 as A068395 is to primes A000040. As with A068395, this is always a multiple of 9, hence cannot be prime. But, as happens first for a(4), a(n) can be semiprime.
FORMULA
a(n) = A001358(n) - A007953(A001358(n)).
EXAMPLE
a(4) = 10 - 1 = 9.
a(5) = 14 - 5 = 9.
MAPLE
read("transforms") :
A206011 := proc(n)
s := A001358(n) ;
s -digsum(s) ;
end proc: # R. J. Mathar, Sep 14 2012
MATHEMATICA
#-Total[IntegerDigits[#]]&/@Select[Range[200], PrimeOmega[#]==2&] (* Harvey P. Dale, Nov 24 2022 *)
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Jonathan Vos Post, Feb 02 2012
STATUS
approved