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Cf. A173919 (Numbers that are prime or one less than a prime.).
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"Rounding to the closest integer" might be seen as is ambiguous for decimal numbers like (k.5) where k is an integer. The tradition wants Here we round such numbers to be rounded to (k+1). This is the case here. Note that the The only occurrence of such a "rounding ambiguity" in the sequence happens with a(1) = 1 and a(2) = 5. Indeed, no more (k.5) "dilemmas" like that one will ever occur again as the integers 50, 500, 5000,... (that might produce together with the previous term k the decimal number k.50 or k.500 or k.5000...) cannot be part of the sequence; this is because 50, 500, 5000,... are not primes themselves (they end with 0) and neither are 51, 501, 5001,... (the can be divided they are divisible by 3).
The assembly [a(1),a(2)] is 1.5 which rounded to the closest integer upwards produces 2;
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"Rounding to the closest integer" might be seen as ambiguous for decimal numbers like (k.5) where k is an integer. The tradition wants such numbers to be rounded to (k+1). This is the case here. Note that the only occurrence of such a "rounding ambiguity" in the sequence happens with a(1) = 1 and a(2) = 5. Indeed, no more (k.5) "dilemmas" like that one will ever occur again as the integers 50, 500, 5000,... (that might produce together with the previous term k the decimal number k.50 or k.500 or k.5000...) cannot be part of the sequence ; this is because 50, 500, 5000,... are not primes themselves (they end with 0) and neither are 51, 501, 5001,... neither (the can be divided by 3).
"Rounding to the closest integer" might be seen as ambiguous for decimal numbers like (k.5) where k is an integer. The tradition wants such numbers to be rounded to (k+1). This is the case here. Note that the only occurrence of such a "rounding ambiguity" in the sequence happens with a(1) = 1 and a(2) = 5. Indeed, no more (k.5) "dilemmas" like that one will ever occur again as the integers 50, 500, 5000,... (that might produce together with the previous term k the decimal number k.50 or k.500 or k.5000...) cannot be part of the sequence becauses because 50, 500, 5000,... are not primes themselves (they end with 0) and so aren't 51, 501, 5001,... neither (the can be divided by 3).
"Rounding to the closest integer" might be seen as ambiguous for decimal numbers like (k.5) where k is an integer. The tradition wants such numbers to be rounded to (k+1). This is the case here. Note that the only occurrence of such a "rounding ambiguity" in the sequence happens with a(1) = 1 and a(2) = 5. But Indeed, no more (k.5) "dilemmas" like this that one will ever occur again as the integers 50, 500, 5000,... (that might produce together with the previous term k the decimal number k.50 or k.500 or k.5000...) cannot be part of the sequence as becauses 50, 500, 5000,... are not primes themselves and so aren't 51, 501, 5001,... neither.