A283561
Numbers k such that the concatenation of the first k nonsquares gives a prime.
Original entry on oeis.org
1, 2, 5, 550, 832, 10431
Offset: 1
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cns[n_]:=FromDigits[Flatten[IntegerDigits[Table[k+Floor[1/2+Sqrt[k]],{k,1,n}]]]]
Select[Table[cns[n],{n,6100}],PrimeQ]
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is(n)=my(s=""); for(k=1,n, s=Str(s, (sqrtint(4*k)+1)\2 + k)); ispseudoprime(eval(s)) \\ Charles R Greathouse IV, Mar 10 2017
A283801
Concatenation of the first n odd composite numbers (A071904).
Original entry on oeis.org
9, 915, 91521, 9152125, 915212527, 91521252733, 9152125273335, 915212527333539, 91521252733353945, 9152125273335394549, 915212527333539454951, 91521252733353945495155, 9152125273335394549515557, 915212527333539454951555763, 91521252733353945495155576365
Offset: 1
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bb[1]=9;bb[n_]:=bb[n]=Which[PrimeQ[bb[n-1]+2]==False,bb[n-1]+2,PrimeQ[bb[n-1]+4]==False,bb[n-1]+4,True,bb[n-1]+6];coc[n_]:=FromDigits[Flatten[IntegerDigits[Table[bb[k],{k,1,n}]]]];Table[coc[n],14]
f[n_] := Block[{oc = cc = 0, k = 2}, While[oc <= n, If[ OddQ@ k && !PrimeQ@ k, cc = cc*10^IntegerLength[k] +k; oc++]; k++]; cc]; Array[f, 14] (* Robert G. Wilson v, Mar 17 2017 *)
Showing 1-2 of 2 results.
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