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 A187886 a(n) = the smallest prime p such that np + 1 is a square, or 0 if no such square exists. 2
 3, 0, 5, 2, 3, 0, 5, 3, 7, 0, 13, 2, 11, 0, 13, 3, 19, 0, 17, 0, 3, 0, 0, 2, 23, 0, 29, 0, 31, 0, 29, 7, 3, 0, 37, 0, 0, 0, 5, 2, 43, 0, 41, 0, 43, 0, 0, 11, 47, 0, 5, 0, 0, 0, 53, 3, 7, 0, 61, 2, 59, 0, 61, 17, 3, 0, 0, 0, 7, 0, 73, 5, 71, 0, 73, 0, 79, 0, 0, 19, 79, 0, 0, 2, 3, 0, 89, 5, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS if n = A007510(k) (Single primes), then a(n) = 0. LINKS EXAMPLE a(6) = 0 because there is no prime p such that 6*p + 1 is a square. a(11) = 13 because 11*13 + 1 = 144 is a square. MAPLE with(numtheory): for k from 1 to 120 do : q:=0:for p from 1 to 200 do : x:=sqrt(k*p+1)   : if x=trunc(x) and type(p, prime)=true and q=0 then q:=1: printf(`%d, `, p):else   fi:od:if q=0 then printf(`%d, `, q):else fi:od: CROSSREFS Cf. A007510, A187884, 187885. Sequence in context: A226770 A185782 A304027 * A324103 A130054 A236146 Adjacent sequences:  A187883 A187884 A187885 * A187887 A187888 A187889 KEYWORD nonn,base AUTHOR Michel Lagneau, Mar 15 2011 STATUS approved

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Last modified August 1 18:47 EDT 2021. Contains 346402 sequences. (Running on oeis4.)