

A342950


7smooth numbers not divisible by 10: positive numbers whose prime divisors are all <= 7 but do not contain both 2 and 5.


3



1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 14, 15, 16, 18, 21, 24, 25, 27, 28, 32, 35, 36, 42, 45, 48, 49, 54, 56, 63, 64, 72, 75, 81, 84, 96, 98, 105, 108, 112, 125, 126, 128, 135, 144, 147, 162, 168, 175, 189, 192, 196, 216, 224, 225, 243, 245, 252, 256, 288, 294, 315, 324
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OFFSET

1,2


LINKS

David A. Corneth, Table of n, a(n) for n = 1..10195


FORMULA

Sum_{n>=1} 1/a(n) = 63/16.  Amiram Eldar, Apr 01 2021


EXAMPLE

12 is in the sequence as all of its prime divisors are <= 7 and 12 is not divisible by 10.


MATHEMATICA

Select[Range@500, Max[First/@FactorInteger@#]<=7&&Mod[#, 10]!=0&] (* Giorgos Kalogeropoulos, Mar 30 2021 *)


PROG

is(n) = if(n%10 == 0, return(0)); forprime(p = 2, 7, n/=p^valuation(n, p)); n==1
(Python)
A342950_list, n = [], 1
while n < 10**9:
if n % 10:
m = n
for p in (2, 3, 5, 7):
q, r = divmod(m, p)
while r == 0:
m = q
q, r = divmod(m, p)
if m == 1:
A342950_list.append(n)
n += 1 # Chai Wah Wu, Mar 31 2021


CROSSREFS

Union of A108319 and A108347.
Intersection of A002473 and A067251.
Sequence in context: A087087 A050742 A290387 * A238985 A337230 A226900
Adjacent sequences: A342947 A342948 A342949 * A342951 A342952 A342953


KEYWORD

nonn


AUTHOR

David A. Corneth, Mar 30 2021


STATUS

approved



