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A202286 Smallest prime which is equal to a multiple of its reversal +- a prime smaller than itself in exactly n ways. 4

%I #46 Jan 17 2020 10:43:45

%S 31,41,71,61,6421,8501,8116001,845534401

%N Smallest prime which is equal to a multiple of its reversal +- a prime smaller than itself in exactly n ways.

%C The sequence is finite, more specifically there cannot be more than 19 terms. Proof: To have p = k*R(p) -+ q, with q < p, we must have 0 < k = (p +- q) / R(p) < 2p / (p/10) = 20, since the prime p cannot end in 0 and therefore R(p) > p/10. (R(p) and p have the same number of digits.) Thus, for a given prime p, there cannot be more than 19 solutions (k=1..19) to p - k*R(p) = +- q, and therefore no a(n) beyond n=19. - _M. F. Hasler_, Mar 13 2012, improved following remarks from _Hans Havermann_, Mar 14 2012

%C A refined analysis shows that the maximal number is less. On one hand, if R(p) is odd, then only even k can yield a prime. Therefore the maximal number of solutions can only be obtained for p starting with an even digit <= 8, and therefore p/R(p) < 9, thus k < 2p/R(p) < 18, k <= 17. Moreover, R(p) is not a multiple of 3 (since p isn't), therefore 1/3 of the k-values lead to q == 0 (mod 3) and are excluded, which leaves at most 11 possibilities. Other k-values lead to q == 0 (mod 5), unless p starts with '5'. In any case there cannot be more than 9 solutions. - _M. F. Hasler_, Mar 14 2012

%C a(9) > 8*10^15, if it exists. - _Giovanni Resta_, Oct 29 2018

%H Claudio Meller, <a href="http://simplementenumeros.blogspot.com/2011/12/833-igual-un-multiplo-del-inverso-mas.html">Números y algo mas...: 833 - Igual a un múltiplo del "inverso" mas/ menos un primo</a>

%e a(4)=61 because 61 = 16 x 2 + 29, 61 = 16 x 3 + 13, 61 = 16 x 4 - 3, and 61 = 16 x 5 - 19.

%o (PARI) A202286(n)={ forprime(p=1,default(primelimit), my(r=A004086(p)); 2*p > n*r & sum(k=1,(2*p-1)\r, isprime(abs(p-k*r)))==n & return(p))} \\ _M. F. Hasler_, Mar 14 2012

%Y Cf. A099180, A209063, A182239.

%K nonn,base,fini,more

%O 1,1

%A _Claudio Meller_, Dec 15 2011, a(3) from _Olivier Gérard_

%E a(7)-a(8) from _Hans Havermann_, Mar 12 2012

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Last modified March 28 16:58 EDT 2024. Contains 371254 sequences. (Running on oeis4.)