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A036311
Composite numbers whose prime factors contain no digits other than 2 and 5.
1
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)
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
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved