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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 Table of n, a(n) for n=1..18. 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 Cf. A135945, A135946. Sequence in context: A198313 A219350 A222511 * A268169 A177241 A076821 Adjacent sequences: A135941 A135942 A135943 * A135945 A135946 A135947 KEYWORD nonn,base,more AUTHOR Giorgio Balzarotti & Paolo P. Lava, Dec 07 2007 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.)