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 A195335 Smallest Xmas tree prime with k digits where k is the n-th triangular number. 2
 2, 211, 211151, 2111511013, 211151101310867, 211151101310867100673, 2111511013108671006731000357, 211151101310867100673100035710000931, 211151101310867100673100035710000931100000213, 2111511013108671006731000357100009311000002131000000901, 211151101310867100673100035710000931100000213100000090110000001797 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A Xmas tree prime is a prime which is a concatenation of a prime with a single digit, a prime with two digits, a prime with three digits, a prime with four digits etc. By definition, the number of digits is a triangular number (A000217). Leading zeros are not allowed for any of the primes. LINKS Terry Trotter, Xmas tree primes MAPLE read("transforms") ; A195335 := proc(n)         option remember;         local prev, nxt, a ;         if n =1 then                 2;         else                 prev := procname(n-1) ;                 for nxt from 10^(n-1) to 10^n-1 do                         if isprime(nxt) then                                 a := digcat2(prev, nxt) ;                                 if isprime(a) then                                         return a ;                                 end if;                         end if;                 end do:                 return -1 ;         end if; end proc: # R. J. Mathar, Sep 20 2011 CROSSREFS Cf. A000217. Sequence in context: A068814 A170920 A215641 * A090560 A181903 A078280 Adjacent sequences:  A195332 A195333 A195334 * A195336 A195337 A195338 KEYWORD nonn,base AUTHOR Kausthub Gudipati, Sep 16 2011 STATUS approved

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