OFFSET
1,1
COMMENTS
Subsequence of A030630, integers whose number of divisors is 12. - Michel Marcus, Nov 11 2015
MATHEMATICA
f[n_]:=Sort[Last/@FactorInteger[n]]=={1, 5}; Select[Range[6000], f] (* Vladimir Joseph Stephan Orlovsky, May 03 2011 *)
With[{nn=50}, Take[Union[Flatten[{#[[1]]^5 #[[2]], #[[1]]#[[2]]^5}&/@Subsets[ Prime[ Range[nn]], {2}]]], nn]] (* Harvey P. Dale, Mar 18 2013 *)
PROG
(PARI) list(lim)=my(v=List(), t); forprime(p=2, (lim\2)^(1/5), t=p^5; forprime(q=2, lim\t, if(p==q, next); listput(v, t*q))); vecsort(Vec(v)) \\ Altug Alkan, Nov 11 2015
(PARI) isok(n)=my(f=factor(n)[, 2]); f==[5, 1]~||f==[1, 5]~
for(n=1, 1e4, if(isok(n), print1(n, ", "))) \\ Altug Alkan, Nov 11 2015
(Python)
from sympy import primepi, primerange, integer_nthroot
def A178740(n):
def bisection(f, kmin=0, kmax=1):
while f(kmax) > kmax: kmax <<= 1
kmin = kmax >> 1
while kmax-kmin > 1:
kmid = kmax+kmin>>1
if f(kmid) <= kmid:
kmax = kmid
else:
kmin = kmid
return kmax
def f(x): return n+x-sum(primepi(x//p**5) for p in primerange(integer_nthroot(x, 5)[0]+1))+primepi(integer_nthroot(x, 6)[0])
return bisection(f, n, n) # Chai Wah Wu, Mar 27 2025
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Will Nicholes, Jun 08 2010
STATUS
approved
