A154390 - OEIS

%I #22 Dec 15 2021 07:49:30

%S 12,15,20,21,22,25,27,30,32,33,35,45,50,51,52,54,55,57,70,72,74,75,76,

%T 77,102,105,112,115,117,120,121,122,123,125,130,132,133,135,145,147,

%U 150,152,153,154,155,170,171,172,174,175,176,177,200,201,202,203,205,207

%N Composites whose largest digit is prime.

%H G. C. Greubel, <a href="/A154390/b154390.txt">Table of n, a(n) for n = 1..1000</a>

%e 12 is a term because it is composite and its largest digit (2) is prime.

%e 15 is a term because it is composite and its largest digit (5) is prime.

%e 20 is a term because it is composite and its largest digit (2) is prime.

%p a := proc (n) local nn; nn := convert(n, base, 10): if isprime(n) = false and isprime(max(seq(nn[i], i = 1 .. nops(nn)))) = true then n else end if end proc: seq(a(n), n = 1 .. 220); # _Emeric Deutsch_, Jan 27 2009

%t Select[Range[250],!PrimeQ[#]&&PrimeQ[Max[IntegerDigits[#]]]&] (* _Harvey P. Dale_, Dec 20 2012 *)

%o (GAP) A:=Filtered([2..210],n->not IsPrime(n));;

%o B:=List(List(List(List(A,ListOfDigits),Reversed),Set),Reversed);;

%o a:=List(Filtered([1..Length(B)],i->IsPrime(B[i][1])),i->A[i]); # _Muniru A Asiru_, Feb 10 2019

%o (Python)

%o from sympy import isprime

%o def ok(n): return max(str(n)) in "2357" and not isprime(n)

%o print([k for k in range(208) if ok(k)]) # _Michael S. Branicky_, Dec 15 2021

%Y Subsequence of A117815.

%Y Cf. A000040, A002808, A054055, A155777.

%K nonn,base

%O 1,1

%A _Juri-Stepan Gerasimov_, Jan 08 2009

%E Corrected by _Juri-Stepan Gerasimov_, Jan 28 2009

%E Corrected (added 21) and extended by _Emeric Deutsch_, Jan 27 2009