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A164987 First pair of primes (p1, p2) that begin centuries of primes having the same prime configuration, ordered by increasing p2. Each configuration is allowed only once. 3
390503, 480803, 351121, 566821, 78901, 578701, 323623, 606223, 326701, 645901, 619471, 745471, 655717, 842617, 437321, 855821, 854713, 876913, 811337, 915437, 561409, 920509, 515401, 956401, 452401, 1023601, 805633, 1049333, 247141, 1092541, 1037903, 1127603 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Rearranged the pairs of numbers so that the sequence of values of p2 increases. The first pair is for the primes 390500 + {3, 27, 39, 53, 81} and 480803 + {3, 27, 39, 53, 81}. There is a large, but finite number of terms. How many terms are there? - T. D. Noe, Feb 10 2013
The sequence lists the small prime twin centuries. As exploration goes into higher primes many are found to be triples or even higher. Example: 1072009 is a twin with 5179509, a triple with 7183109, quadruple with 8284709, quintuple with 8462609, and sextuple with 9739309, and there could be infinitely more. - Ki Punches, Dec 17 2009
The first two centuries without any primes start with 1671800 and 2637800. These are not included in the sequence since they do not have a first prime. However, if they were to be included they would be the 136th pair. - Andrew Howroyd, Feb 25 2018
LINKS
EXAMPLE
The primes in 480800..480899 are 480803, 480827, 480839, 480853, 480881 ending with 03, 27, 39, 53, 81. The primes in 390500..390599 end with the same digits, and no earlier pair has this quality. Hence a(1) = 390503 and a(2) = 480803.
MATHEMATICA
pSig[n_] := Prime[Range[PrimePi[100 n] + 1, PrimePi[100 (n + 1)]]] - 100 n; t = {}; c = {}; found = {}; Do[s = pSig[n]; If[Length[s] > 0 && ! MemberQ[found, s] && MemberQ[c, s], d = Mod[s[[1]], 100]; AppendTo[found, s]; AppendTo[t, {Position[c, s][[1, 1]]*100 + d, n*100 + d}]]; AppendTo[c, s], {n, 11000}]; Flatten[t] (* T. D. Noe, Feb 10 2013 *)
PROG
(PARI)
sig(c)={my(s=0); for(v=0, 49, if(isprime(100*c+2*v+1), s+=2^v)); s}
pairs(n)={my(L=List(), M=Map(), c=0); while(#L<2*n, c++; my(s=sig(c), f=0); if(mapisdefined(M, s, &f), if(f&&s, my(d=2*valuation(s, 2)+1); listput(L, 100*f+d); listput(L, 100*c+d); mapput(M, s, 0)), mapput(M, s, c))); Vec(L)}
pairs(20) \\ Andrew Howroyd, Feb 25 2018
CROSSREFS
Cf. A038822 (number of primes between 100n and 100n+99).
Cf. A181098, A186393-A186408 (centuries having 0 to 16 primes).
Sequence in context: A233491 A319505 A106778 * A176765 A290499 A016820
KEYWORD
base,nonn,fini
AUTHOR
Ki Punches, Sep 03 2009 through Dec 06 2009
EXTENSIONS
Terms rearranged by T. D. Noe, Feb 10 2013
STATUS
approved

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Last modified April 16 10:45 EDT 2024. Contains 371709 sequences. (Running on oeis4.)