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 A168513 Number of 2n-digit primes that are concatenation of n two-digit distinct primes p_1...p_n, 98>p_1>p_2>...>p_n>10. 2
 21, 36, 300, 722, 2493, 4766, 9459, 13091, 18484, 18779, 18048, 12995, 9122, 4424, 2160, 639, 234, 48, 8, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS There are 21 two-digit primes, hence n=1..21, and a(1)=21. Also a(2)=36, see A168499, and a(3)=300, see A168500; a(20)=0 because among 21 twenty-digit numbers no one is prime; a(21)=0 because the only possible 42-digit number is composite: 978983797371676159534743413731292319171311= 2633*561934423224622169*661666288057389220943. LINKS MATHEMATICA pr2={97, 89, 83, 79, 73, 71, 67, 61, 59, 53, 47, 43, 41, 37, 31, 29, 23, 19, 17, 13, 11}; Table[ssk=Subsets[pr2, {k}]; Le=Binomial[21, k]; c=0; Do[If[PrimeQ[fd=FromDigits[Flatten[IntegerDigits/@ssk[[i]]]]], c++ ], {i, Le}]; {k, c}, {k, 21}] {1, 21}, {2, 36}, {3, 300}, {4, 722}, {5, 2493}, {6, 4766}, {7, 9459}, {8, 13091}, {9, 18484}, {10, 18779}, {11, 18048}, {12, 12995}, {13, 9122}, {14, 4424}, {15, 2160}, {16, 639}, {17, 234}, {18, 48}, {19, 8}, {20, 0}, {21, 0}. CROSSREFS Cf. A168499, A168500. Sequence in context: A001491 A112352 A298472 * A067598 A043683 A173589 Adjacent sequences:  A168510 A168511 A168512 * A168514 A168515 A168516 KEYWORD base,fini,full,nonn AUTHOR Zak Seidov, Nov 28 2009 STATUS approved

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Last modified October 29 04:54 EDT 2020. Contains 338066 sequences. (Running on oeis4.)