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 A250312 Numbers which produce primes if their divisors, one by one, are prepended, inserted or appended. 3
 1, 151, 157, 169, 223, 277, 283, 337, 361, 367, 397, 409, 421, 439, 457, 469, 547, 571, 577, 589, 607, 643, 673, 709, 757, 769, 793, 871, 877, 937, 1063, 1093, 1201, 1603, 1609, 1807, 2029, 2053, 2071, 2707, 3019, 3037, 3049, 3073, 3109, 3607, 4039, 4051, 4087 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Paolo P. Lava, Table of n, a(n) for n = 1..1000 EXAMPLE Divisors of 1 is 1 and concat(1,1) = 11 is prime. Divisors of 151 are 1, 151. Then concat(151,1) = 1511 is prime, as is concat(1,151) = 1151, and concat(1,151,51) = 115151 is prime. Divisors of 169 are 1, 13, 169. Then concat(16,1,9) = 1619 is prime, concat(16,13,9) = 16139 is prime, as is concat(1,13,69) = 11369, and concat(1,169,69) = 116969 is prime. MAPLE with(numtheory): P:=proc(q) local a, b, c, f, g, h, j, k, n; for n from 1 by 2 to q do a:=divisors(n); h:=0; for k from 1 to nops(a) do b:=ilog10(a[k])+1; for j from 0 to ilog10(n)+1 do f:=(n mod 10^j); if j=0 then c:=n*10^b+a[k]; else g:=a[k]*10^(ilog10(f)+1)+f; c:=trunc(n/10^j)*10^(ilog10(g)+1)+g; fi; if isprime(c) then h:=h+1; break; fi; od; if h=nops(a) then print(n); fi; od; od; end: P(10^6); CROSSREFS CF. A250311. Sequence in context: A115483 A182875 A336777 * A180554 A243531 A139505 Adjacent sequences: A250309 A250310 A250311 * A250313 A250314 A250315 KEYWORD nonn,base,easy AUTHOR Paolo P. Lava, Nov 18 2014 EXTENSIONS Inserted a(3), a(16) and a(26) by Paolo P. Lava, Nov 21 2014 STATUS approved

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Last modified November 30 12:26 EST 2023. Contains 367461 sequences. (Running on oeis4.)