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A266146 Number of n-digit primes in which n-1 of the digits are 7's. 11
4, 8, 10, 9, 12, 11, 8, 4, 9, 9, 10, 14, 14, 11, 16, 7, 10, 17, 7, 10, 9, 12, 9, 13, 11, 10, 14, 5, 3, 22, 6, 13, 13, 10, 8, 16, 8, 6, 16, 8, 13, 14, 8, 7, 8, 13, 9, 11, 13, 9, 14, 8, 4, 23, 13, 11, 8, 8, 8, 12, 13, 13, 11, 11, 10, 23, 11, 8, 8, 3, 6, 16, 12, 13, 12, 12, 8, 11, 8, 11, 14, 13, 7, 15, 12, 17, 11, 7, 9, 21, 6, 6, 11, 12, 6, 14, 14, 12, 13, 12, 11, 17, 10, 17, 18 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1215
EXAMPLE
a(2) = 8 from 17, 37, 47, 67, 71, 73, 79, 97. - N. J. A. Sloane, Dec 27 2015
a(3) = 10 since 277, 577, 677, 727, 757, 773, 787, 797, 877, and 977 are primes.
MATHEMATICA
f7[n_] := Block[{cnt = k = 0, r = 7 (10^n - 1)/9, s = Range[0, 9] - 7}, While[k < n, cnt += Length@ Select[r + 10^k*s, PrimeQ@ # && IntegerLength@ # > k &]; k++]; cnt]; Array[f7, 100]
PROG
(PARI) a(n)={sum(i=0, n-1, sum(d=i==n-1, 9, isprime((10^n-1)/9*7 + (d-7)*10^i)))} \\ Andrew Howroyd, Feb 28 2018
(Python)
from __future__ import division
from sympy import isprime
def A266146(n):
return 4*n if (n==1 or n==2) else sum(1 for d in range(-7, 3) for i in range(n) if isprime(7*(10**n-1)//9+d*10**i)) # Chai Wah Wu, Dec 27 2015
CROSSREFS
Sequence in context: A305372 A261602 A265733 * A329503 A108806 A310967
KEYWORD
base,nonn
AUTHOR
EXTENSIONS
a(2) corrected by Chai Wah Wu, Dec 27 2015
a(2) corrected in b-file as above by Andrew Howroyd, Feb 28 2018
STATUS
approved

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Last modified March 19 06:17 EDT 2024. Contains 370952 sequences. (Running on oeis4.)