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 A215647 Smallest semiprime whose decimal expansion consists of the concatenation of a 1-digit semiprime, a 2-digit semiprime, a 3-digit semiprime, ..., and an n-digit semiprime. 10
 4, 415, 410111, 4101061003, 410106100310003, 410106100310001100001, 4101061003100011000011000057, 410106100310001100001100000110000051, 410106100310001100001100000110000001100000001 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is to A215641 as semiprimes A001358 are to primes A000040. It is a plausible conjecture that a(n) always exists. If a(n) exists it has A000217(n) = n(n+1)/2 digits. LINKS EXAMPLE a(2) = 415 = 5 * 83 = concatenation of 4 and 15, where 4 is the semiprime 2^2 and 15 is the semiprime 3 * 5. a(3) = 410111 = 13 * 31547 = the concatenation of 4, 10, and 111 where 4 = 2^2, 10 = 2 * 5, and 111 = 3 * 37. a(5) = 410106100310003 = 76871 * 5334991093 = Concatenate(4,10,106,1003,10003). CROSSREFS Cf. A001358, A215641. Sequence in context: A198709 A326209 A287965 * A228242 A036771 A201980 Adjacent sequences:  A215644 A215645 A215646 * A215648 A215649 A215650 KEYWORD nonn,base AUTHOR Jonathan Vos Post, Aug 19 2012 STATUS approved

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Last modified May 9 06:30 EDT 2021. Contains 343692 sequences. (Running on oeis4.)