A242202 -id:A242202 - cp's OEIS frontend

1, 18, 44, 60, 80, 123, 429, 1166, 2065, 8774, 35340, 42968, 50312, 210129

Offset: 1

a(15) > 400000.

Numbers n such that n * b^n - 1 is prime: A008864 (b=1), A002234 (b=2), A006553 (b=3), A086661 (b=4), A059676 (b=5), A059675 (b=6), A242200 (b=7), A242201 (b=8), A242202 (b=9), A059671 (b=10), A299374 (b=11), A299375 (b=12), A299376 (b=13), A299377 (b=14), A299378 (b=15), A299379 (b=16), A299380 (b=17), A299381 (b=18), A299382 (b=19), this sequence (b=20).

[n: n in [1..10000] |IsPrime(n*20^n-1)];

Select[Range[1, 10000], PrimeQ[n*20^n-1] &]

for(n=1, 10000, if(isprime(n*20^n-1), print1(n", ")))

2, 3, 6, 8, 11, 18, 28, 38, 48, 168, 254, 274, 310, 430

Offset: 1

Vincenzo Librandi, May 12 2014

The semiprimes of this form are: 161, 2186, 3188645, 344373767, 345191655698, 2701703435345984177, 14653733724766095041978322587, ...

a(15) >= 512. - Daniel Suteu, Aug 05 2019

Cf. similar sequences listed in A242273.

IsSemiprime:=func; [n: n in [2..70] | IsSemiprime(s) where s is n*9^n-1];

Mathematica

Select[Range[70], PrimeOmega[# 9^# - 1]==2&]

Extensions

a(10)-a(14) from Daniel Suteu, Aug 05 2019

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