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 A037053 Smallest prime containing exactly n 0's. 21
 2, 101, 1009, 10007, 100003, 1000003, 20000003, 100000007, 1000000007, 30000000001, 100000000003, 2000000000003, 40000000000001, 1000000000000037, 6000000000000001, 20000000000000003, 100000000000000003, 1000000000000000003, 60000000000000000007, 500000000000000000003 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Certain entries require n+3 digits such as a(13). Those which do not require three digits besides n zeros are in A085824. Conjecture: a prime of this form, A037053, requires at most three judiciously placed nonzero digits, two on the exterior and one inside. - Robert G. Wilson v The zeros in a(n) do not need to be consecutive. If the zeros must be consecutive we get a new sequence which agrees with this one up though a(31), but then here we have a(32) = 10000000000000000000000000000000603, whereas the smallest prime with exactly 32 consecutive zeros is 19000000000000000000000000000000009 = 19*10^33+9. - N. J. A. Sloane, Feb 20 2016 Sequence A269230 lists the indices (32, 43, 46, 49, 50, 60, 69, ...) for which a(n) does not have n consecutive digits '0', and gives more information about the smallest prime which has n consecutive digits '0': Sequence A269260 lists the values a > 9 such that the least prime with n *consecutive* '0's equals nextprime(a*10^(n+1)), for these indices n. - M. F. Hasler, Feb 20 2016 and Feb 22 2016 Since the definition requires "exactly" and not "at least" n 0's, the sequence is not increasing, e.g., a(22) = 10^24 + 49 > a(23) = 10^24 + 7. However, it seems that no term has more digits than its successor. - M. F. Hasler, Feb 20 2016 From Bob Selcoe, Feb 20 2016: (Start) Conjecture 1: (following Robert G. Wilson v's conjecture above and insight provided by Hans Havermann): terms with two substrings of zeros have first digit of 1. Conjecture 2: as n increases, there are more terms with two substrings of zeros than one consecutive string. The logic is as follows: Let n = number of zeros and z = number of substrings of consecutive zeros contained in candidate primes. Candidates with two substrings of zeros (z=2) must be considered after only 72 smaller z=1 candidates have been considered and excluded, i.e., numbers of the form a[n zeros]b and 1[n zeros]ab, where a in {1..9}, b in {1,3,7,9}. After these 72 candidates are excluded, 36*n z=2 candidates are considered before having to consider only 36 additional z=1 candidates (i.e., 2[n zeros]ab), followed by 36*n additional z=2 candidates, etc. So as n increases, it becomes increasingly unlikely that any z=1 term appears. Additionally, the number of candidates increases as n increases for z>=3. For a given n, 1044 + 324*(n+1) candidates must be excluded before considering the smallest z=3: 1[n-2 zeros]10101. Since the probability p of n-digit primes occurring is p ~ 1/(n log 10) by the Prime Number Theorem, and the number of z in {1,2} candidates which must be excluded increases exponentially as n and z increase, it is unlikely that any z >= 3 term appears in A037053 and that first digit will be > 1 for any z=2 term. Sequence A269233 lists the number of candidate primes < a(n); i.e., the number of excluded candidates. (End) LINKS Robert Israel and Robert G. Wilson v, Table of n, a(n) for n = 0..1000 a(0) - a(900) from Robert Israel. Hans Havermann, Compactly expressed table of n, a(n) for a large number of n MAPLE F:= proc(n) local a, b, cands, p; cands:= [seq(seq(10^(n+1)*a+b, b=[1, 3, 7, 9]), a=1..9), seq(seq(seq(10^(n+2)+a*10^j+b, b=[1, 3, 7, 9]), a=1..9), j=1..n+1)]; for p in cands do if isprime(p) then return p fi od; error("No candidate is prime"); end proc: 2, seq(F(n), n=1..40); # Robert Israel, Feb 19 2016 MATHEMATICA f[n_] := Block[{pc}, a = 1; While[a < 10, b = 1; While[b < 10, pc = a*10^(n + 1) + b; If[PrimeQ[pc], Goto[fini]]; b += 2]; a++]; e = 1; While[e < n + 2, b = 1; While[b < 10, c = 1; While[c < 10, pc = 10^(n + 2) + b*10^e + c; If[ PrimeQ[pc], Goto[ fini]]; c += 2]; b++]; e++]; Label[ fini]; pc]; f = 2; Array[f, 25, 0] (* Robert G. Wilson v, Feb 21 2016 *) PROG (PARI) A037053(n)={n&&forstep(i=n=10^(n+1), 9*n, n, nextprime(i)p*=10, forstep(j=i+p, i+9*p, p, nextprime(j)

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Last modified October 22 22:34 EDT 2019. Contains 328335 sequences. (Running on oeis4.)