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A036312 Composite numbers whose prime factors contain no digits other than 2 and 7. 2
4, 8, 14, 16, 28, 32, 49, 56, 64, 98, 112, 128, 196, 224, 256, 343, 392, 448, 454, 512, 554, 686, 784, 896, 908, 1024, 1108, 1372, 1454, 1568, 1589, 1792, 1816, 1939, 2048, 2216, 2401, 2744, 2908, 3136, 3178, 3584, 3632, 3878, 4096, 4432, 4802, 5089, 5488 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020459. - David A. Corneth, Oct 09 2020
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000 (first 5340 terms from Robert Israel)
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A020459} (p/(p - 1)) - Sum_{p in A020459} 1/p - 1 = 0.7041098484... . - Amiram Eldar, May 18 2022
MAPLE
dmax:= 4: # for terms < 2*10^dmax
P:= {2, 7}:
L:= {7}:
for d from 1 to dmax-1 do
L:= map(t -> 2*10^d+t, L) union map(t -> 7*10^d+t, L);
P:= P union select(isprime, L);
od:
R:= {1}: N:= 2*10^dmax:
for p in P do
R:= R union map(t -> seq(t*p^j, j=1..floor(log[p](N/t))), R)
od:
sort(convert(R minus P minus {1}, list)); # Robert Israel, Aug 04 2020
CROSSREFS
Sequence in context: A331871 A331965 A331683 * A312338 A312339 A024805
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved

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Last modified June 21 03:10 EDT 2024. Contains 373535 sequences. (Running on oeis4.)