A206011 The n-th semiprime minus its sum of digits.

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

Author

Jonathan Vos Post, Feb 02 2012

Keywords

• nonn
• easy
• base

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.

Examples

			a(4) = 10 - 1 = 9.
a(5) = 14 - 5 = 9.
		

Crossrefs

Cf. A000040, A001358, A007953, A068395.

Programs

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 *)

Formula

a(n) = A001358(n) - A007953(A001358(n)).