A036311 Composite numbers whose prime factors contain no digits other than 2 and 5.

4, 8, 10, 16, 20, 25, 32, 40, 50, 64, 80, 100, 125, 128, 160, 200, 250, 256, 320, 400, 500, 512, 625, 640, 800, 1000, 1024, 1250, 1280, 1600, 2000, 2048, 2500, 2560, 3125, 3200, 4000, 4096, 5000, 5120, 6250, 6400, 8000, 8192, 10000, 10240, 12500, 12800

Offset

1,1

Comments

A003592 with 1, 2 and 5 removed. - Robert Israel, Apr 29 2018

Links

Robert Israel, Table of n, a(n) for n = 1..10000 (first 131 terms from Vincenzo Librandi)

Index entries for sequences related to prime factors.

Formula

Sum_{n>=1} 1/a(n) = 4/5. - Amiram Eldar, May 18 2022~

Maple

N:= 20000: # to get all terms <= N
S:= {seq(seq(2^i*5^j, i=0..ilog2(N/5^j)), j=0..floor(log[5](N)))} minus {1, 2, 5}:
sort(convert(S, list)); # Robert Israel, Apr 29 2018

Mathematica

dpfQ[n_]:=Module[{d=Union[Flatten[IntegerDigits/@Transpose[FactorInteger[n]][[1]]]]}, !PrimeQ[n]&&(d == {2}||d == {5}||d == {2, 5})]; Select[Range[15000], dpfQ] (* Vincenzo Librandi, Aug 25 2013 *)

Prog

(Magma) [n: n in [4..13000] | not IsPrime(n) and forall{f: f in PrimeDivisors(n) | Intseq(f) subset [2, 5]}]; // Bruno Berselli, Aug 26 2013

Crossrefs

Cf. A003592, A036302-A036325, A025612.

Sequence in context: A311003 A311004 A144570 * A020209 A311005 A311006

Adjacent sequences: A036308 A036309 A036310 * A036312 A036313 A036314

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Status

approved