OFFSET
1,2
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..1000 (terms 1..30 from N. J. A. Sloane)
FORMULA
a(n) ~ 10^n * 0.10010101.... [Charles R Greathouse IV, Dec 27 2011]
EXAMPLE
A005171 : 1,0,0,1,0,1,0,1,1,1,0,1,0,1,1,1,0,1,0,1,1,1,0,1,1 ................
a(1)=1, a(2)=10, a(3)=100, a(4)=1001, ...
MATHEMATICA
Array[FromDigits@ Array[Boole[! PrimeQ@ #] &, #] &, 21] (* or *)
FromDigits@ IntegerDigits[#, 2] & /@ Last@ Transpose@ NestList[{#1 + 1, If[PrimeQ[#1 + 1], 2 #2, 2 #2 + 1]} & @@ # &, {1, 1}, 21] (* Michael De Vlieger, Nov 01 2016, latter after Harvey P. Dale at A118255 *)
PROG
(PARI) a(n) = sum(k=1, n, !isprime(k)*10^(n-k)); \\ Michel Marcus, Nov 01 2016
(Python)
from sympy import isprime
def a(n): return int("".join(str(1-isprime(i)) for i in range(1, n+1)))
print([a(n) for n in range(1, 22)]) # Michael S. Branicky, Jan 10 2022
(Python) # faster version for initial segment of sequence
from sympy import isprime
from itertools import count, islice
def agen(): # generator of terms
an = 0
for k in count(1):
an = 10 * an + int(not isprime(k))
yield an
print(list(islice(agen(), 21))) # Michael S. Branicky, Jan 10 2022
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Pierre CAMI, Apr 19 2006
EXTENSIONS
Corrected by Omar E. Pol, Nov 08 2007
STATUS
approved