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A135944
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Number of n-digit primes whose digits are all primes.
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2
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4, 4, 15, 38, 128, 389, 1325, 4643, 16623, 59241, 214432, 781471, 2884201, 10687480
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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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. Aproximation of the sum of the sequence up to a(k) for large values of k: sum(a(i), i=1..k) =10 ^ k / ( k *log(10))* ( 2/5)^(k-1).
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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.
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CROSSREFS
| Sequence in context: A048282 A068592 A174406 * A177241 A076821 A165825
Adjacent sequences: A135941 A135942 A135943 * A135945 A135946 A135947
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KEYWORD
| nonn,base
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AUTHOR
| Giorgio Balzarotti & Paolo P. Lava (greenblue(AT)tiscali.it), Dec 07 2007
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EXTENSIONS
| a(12)-a(14) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Feb 05 2010
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