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 A117388 a(n) is the smallest n-digit integer such that, if all numbers formed by inserting the exponentiation symbol between any two digits are added up, the sum is prime. 2
 21, 121, 1226, 14423, 111334, 1186896 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS No zeros are allowed in the decimal representation of a(n). LINKS EXAMPLE a(5) = 14423 since 1^4423+14^423+144^23+1442^3 is prime. MATHEMATICA (* first do *) Needs["DiscreteMath`Combinatorica`"] (* then *) f[n_] := Block[{k = (10^n - 1)/9}, While[id = IntegerDigits@k; First@ Union@ id == 0 || !PrimeQ[Plus @@ Table[FromDigits@ Take[id, {1, k}]^FromDigits@ Take[id, {k + 1, n}], {k, n - 1}]], k++ ]; k]; Do[Print[f[n]] // Timing, {n, 2, 7}] (* Robert G. Wilson v, Apr 27 2006 *) PROG (Python) from sympy import isprime from itertools import product def a(n):     for p in product("123456789", repeat=n):         s = "".join(p)         if isprime(sum(int(s[:i])**int(s[i:]) for i in range(1, n))):             return int(s) print([a(n) for n in range(2, 6)]) # Michael S. Branicky, Jun 27 2022 CROSSREFS Cf. A113762. Sequence in context: A179441 A164785 A179956 * A053052 A243882 A002299 Adjacent sequences:  A117385 A117386 A117387 * A117389 A117390 A117391 KEYWORD base,more,nonn AUTHOR Ray G. Opao, Apr 25 2006 EXTENSIONS a(6) from Robert G. Wilson v and Farideh Firoozbakht, Apr 27 2006 a(7) from Sean A. Irvine, Dec 15 2009 STATUS approved

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Last modified August 10 03:38 EDT 2022. Contains 356029 sequences. (Running on oeis4.)