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A135944 Number of n-digit primes whose digits are all primes. 2
4, 4, 15, 38, 128, 389, 1325, 4643, 16623, 59241, 214432, 781471, 2884201, 10687480, 39838489, 149172297, 560795383, 2115430020 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Number of primes of the form: ...d3d2d1d0 = d0 * 10 ^ 0 + d1 * 10 ^ 1 + d2 * 10 ^ 2 + d3 * 10 ^ 3 + ... where d0, d1, d2, d3, ... are primes with one digit. The i-th element of the sequence is the number of primes with i digits. Approximation of the sum of the sequence up to a(k) for large values of k: Sum_{i=1..k} a(i) = 10^k / (k*log(10))*(2/5)^(k-1).
LINKS
EXAMPLE
a(4) = 38 because there are 38 numbers of the form d3d2d1d0 with d0, d1, d2, d3 prime numbers, namely, 2237, 2273, 2333, 2357, 2377, 2557, ..., 7753, 7757.
CROSSREFS
Sequence in context: A198313 A219350 A222511 * A268169 A177241 A076821
KEYWORD
nonn,base,more
AUTHOR
EXTENSIONS
a(12)-a(14) from Donovan Johnson, Feb 05 2010
a(15)-a(16) from Chai Wah Wu, Nov 28 2015
a(17)-a(18) from Giovanni Resta, May 25 2016
STATUS
approved

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Last modified June 21 21:50 EDT 2024. Contains 373559 sequences. (Running on oeis4.)