A154389 - OEIS

%I #25 Feb 12 2019 01:18:15

%S 1,9,10,39,49,69,90,91,92,93,94,95,96,98,99,100,110,111,119,129,159,

%T 169,189,190,192,194,195,196,198,209,219,249,259,279,289,290,291,292,

%U 294,295,296,297,298,299,309,319,329,339,369,390,391,392,393,394,395,396

%N Nonprimes whose largest digit is an odd nonprime.

%C I.e., nonprimes whose largest digit is 1 or 9. - _Jon E. Schoenfield_, Feb 11 2019

%H G. C. Greubel, <a href="/A154389/b154389.txt">Table of n, a(n) for n = 1..1000</a>

%e 1 is a term because it is nonprime and its largest digit, 1, is an odd nonprime.

%e 9 is a term because it is nonprime and its largest digit, 9, is an odd nonprime.

%e 10 is a term because it is nonprime and its largest digit, 1, is an odd nonprime.

%e 39 is a term because it is nonprime and its largest digit, 9, is an odd nonprime.

%p A141468 := proc(n) option remember; if n <= 3 then op(n,[0,1,4]) ; else for a from procname(n-1)+1 do if not isprime(a) then return a; end if; end do: end if; end proc:

%p A054055 := proc(n) max( op(convert(n,base,10)) ) ; end proc:

%p for n from 1 to 500 do c := A141468(n) ; if A054055(c) in {1,9} then printf("%d,",c ) ; end if; end do: # _R. J. Mathar_, May 05 2010

%t ldQ[n_] := Module[{midn = Max[IntegerDigits[n]]}, OddQ[midn] && !PrimeQ[midn]]

%t Select[Complement[Range[450], Prime[Range[PrimePi[450]]]], ldQ] (* _Harvey P. Dale_, Jan 31 2011 *)

%o (PARI) isok(n) = !isprime(n) && (d = vecmax(digits(n))) && (d % 2) && ! isprime(d); \\ _Michel Marcus_, Sep 16 2016

%Y Cf. A054055 (largest digit).

%Y Cf. A141468 (0 together with the nonprime numbers).

%K nonn,base

%O 1,2

%A _Juri-Stepan Gerasimov_, Jan 08 2009

%E Corrected (69 inserted, 111 inserted, 189 inserted, ...) by _R. J. Mathar_, May 05 2010

%E Example section edited by _Jon E. Schoenfield_, Feb 11 2019