A359124
Concatenate the decimal numbers 1,2,3,...,n, then add 1.
Original entry on oeis.org
2, 13, 124, 1235, 12346, 123457, 1234568, 12345679, 123456790, 12345678911, 1234567891012, 123456789101113, 12345678910111214, 1234567891011121315, 123456789101112131416, 12345678910111213141517, 1234567891011121314151618, 123456789101112131415161719, 12345678910111213141516171820
Offset: 1
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Table[FromDigits[Flatten[IntegerDigits/@Range[n]]]+1,{n,20}] (* Harvey P. Dale, Dec 20 2022 *)
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def a(n): return int("".join(map(str, range(1, n+1)))) + 1
print([a(n) for n in range(1, 20)]) # Michael S. Branicky, Dec 20 2022
A359125
Largest prime factor of A359124(n).
Original entry on oeis.org
2, 13, 31, 19, 6173, 123457, 154321, 333667, 333667, 3388877, 4281283, 2630197, 26798700427, 8663199947, 2523244037, 12873492085621702963, 32929947197382727, 17539959825403, 71595329159622797, 325339942922532262019, 9999103057380477280607, 17465868005034957301, 1423364280511
Offset: 1
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FactorInteger[#][[-1,1]]&/@Table[FromDigits[Flatten[IntegerDigits/@Range[n]]]+1,{n,30}] (* Harvey P. Dale, Dec 20 2022 *)
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from sympy import factorint
def a(n): return max(factorint(int("".join(map(str, range(1, n+1)))) + 1))
print([a(n) for n in range(1, 24)]) # Michael S. Branicky, Dec 20 2022
A387245
Numbers k such that (the concatenation of the first k positive even numbers) + 1 is prime.
Original entry on oeis.org
1, 5, 9, 24, 1400, 8915
Offset: 1
5 is a term since 246810 + 1 = 246811 is prime.
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