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 A308438 a(n) is the smallest prime p whose decimal expansion begins with n and is such that the next prime is p+2n, or -1 if no such prime exists. 1
 11, 223, 31, 401, 547, 619, 773, 8581, 9109, 10223, 1129, 12073, 130553, 14563, 150011, 161471, 17257, 18803, 191189, 20809, 210557, 225383, 237091, 240209, 2509433, 2613397, 277429, 283211, 2901649, 308153, 313409, 3204139, 3300613, 3419063, 3507739, 360091, 3727313, 3806347, 3930061, 4045421, 41018911 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Chai Wah Wu, Table of n, a(n) for n = 1..100 EXAMPLE For n = 5, 547 is a prime starting with 5, and the next prime after 547 is 557 = 547 + 2*5.  Since this is the least number with these properties, a(5) = 547. MAPLE f:= proc(n) local d, p, q;   for d from 0 do     p:= nextprime(n*10^d-1);     do       q:= nextprime(p);       if q - p = 2*n then return p fi;       if q >= (n+1)*10^d then break fi;       p:= q;     od;   od; end proc: map(f, [\$1..50]); PROG (Python) from sympy import nextprime def A308438(n):     l, p = 1, nextprime(n)     while True:         q = nextprime(p)         if q-p == 2*n:             return p         p = q         if p >= (n+1)*l:             l *= 10             p = nextprime(n*l) # Chai Wah Wu, May 31 2019 CROSSREFS Cf. A018800, A030665. Sequence in context: A048377 A192686 A323416 * A103611 A142541 A295542 Adjacent sequences:  A308435 A308436 A308437 * A308439 A308440 A308441 KEYWORD nonn,base,look AUTHOR J. M. Bergot and Robert Israel, May 30 2019 STATUS approved

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Last modified January 20 14:17 EST 2021. Contains 340302 sequences. (Running on oeis4.)