OFFSET
1,1
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..5000
MATHEMATICA
Select[ Table[ Prime[ n ], {n, 1000} ], First[ IntegerDigits[ # ]]==7& ]
PROG
(Magma) [p: p in PrimesUpTo(7300) | Intseq(p)[#Intseq(p)] eq 7]; // Vincenzo Librandi, Aug 08 2014
(Python)
from itertools import chain, count, islice
from sympy import primerange
def A045713_gen(): # generator of terms
return chain.from_iterable(primerange(7*(m:=10**l), m<<3) for l in count(0))
(Python)
from sympy import primepi
def A045713(n):
def bisection(f, kmin=0, kmax=1):
while f(kmax) > kmax: 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+primepi(min(7*(m:=10**(l:=len(str(x))-1))-1, x))-primepi(min((m<<3)-1, x))+sum(primepi(7*(m:=10**i)-1)-primepi((m<<3)-1) for i in range(l))
return bisection(f, n, n) # Chai Wah Wu, Dec 08 2024
CROSSREFS
KEYWORD
nonn,base
AUTHOR
EXTENSIONS
More terms from Erich Friedman.
Corrected by Jud McCranie, Jan 03 2001
a(13)=757 added from Vincenzo Librandi, Aug 08 2014
STATUS
approved